UNITED NATIONS (CNN) -- Iran is prepared to engage in nuclear talks and nuclear weapons have no place in his country's security, Iranian President Hassan Rouhani said Tuesday.
In a speech at the U.N. General Assembly, he said that Iran would be willing to "engage immediately in time-bound and result-oriented talks to build mutual confidence and removal of mutual uncertainties."
"Nuclear weapons and other weapons of mass destruction have no place in Iran's security and defense doctrine, and contradict our fundamental religious and ethical convictions," Rouhani said.
He decried international sanctions against Iran.
"Sanctions, beyond any and all rhetoric, cause belligerence, war-mongering and human suffering," he said.
"Iran seeks to resolve problems, not to create them," Rouhani said, pushing for the "rejection of violence and extremism."
Alongside fears facing the world, there are "new hopes," he said. "The hope of universal acceptance and the elite all across the globe of yes to peace and no to war. And the hope of preference of dialogue over conflict, and moderation over extremism."
Striking a conciliatory tone, Rouhani said that Iran "does not seek to increase tensions with the United States."
Rouhani said he listened carefully to U.S. President Barack Obama's speech and hoped that the United States "will refrain from following the short-sighted interests of warmongering pressure groups" so that the two nations "can arrive at a framework to manage our differences."
Earlier Tuesday, two senior administration officials said that Obama and Rouhani wouldn't be shaking hands or meet on the sidelines of the U.N. General Assembly.
Such an encounter proved too complicated for Iran back home, the officials told reporters.
Both presidents spoke on the first day of the annual gathering of world leaders in New York. Obama made clear in his morning remarks that the United States was committed to preventing Iran from developing a nuclear weapon.
"We will not tolerate the development or use of weapons of mass destruction," he said.
At the same time, Obama welcomed what he called positive signals from Iran that it was ready to negotiate with the international community on how it can develop a peaceful use of nuclear power without creating any weapons.
"We are not seeking regime change, and we respect the right of the Iranian people to access peaceful nuclear energy," Obama said. "Instead, we insist that the Iranian government meet its responsibilities under the Nuclear Non-Proliferation Treaty and U.N. Security Council resolutions."
He noted that Iranian Supreme Leader Ayatollah Ali Khamenei issued a fatwa against the development of nuclear weapons, and Rouhani "just recently reiterated that the Islamic Republic will never develop a nuclear weapon."
"These statements made by our respective governments should offer the basis for a meaningful agreement," Obama said, adding that "to succeed, conciliatory words will have to be matched by actions that are transparent and verifiable."
Israeli Prime Minister Benjamin Netanyahu welcomed those comments, but said that his country "will not be fooled by half-measures that merely provide a smokescreen for Iran's continual pursuit of nuclear weapons."
"Iran thinks that soothing words and token actions will enable it to continue on its path to the bomb. Like North Korea before it, Iran will try to remove sanctions by offering cosmetic concessions, while preserving its ability to rapidly build a nuclear weapon at a time of its choosing," Netanyahu said.
Secretary of State John Kerry will join his Iranian counterpart, Foreign Minister Mohammad Javad Zarif, at a Thursday meeting of the five permanent members of the U.N. Security Council, plus Germany. Discussions will surround restarting talks on Iran's nuclear program.
One European Union official expressed optimism over the chance for concrete progress.
"In terms of whether we're on the verge of a breakthrough, I would put it like this: I was struck as I said by the energy and determination the foreign minister demonstrated to me," said Catherine Ashton, high representative for foreign affairs and security policy of the European Union.
But no one is expecting an overnight solution to Iran's alleged effort to build a nuclear weapon, an effort Tehran has so far denied, insisting its nuclear program is for peaceful purposes.
"The roadblocks may prove to be too great, but I firmly believe the diplomatic path must be tested," Obama said.
Syria a point of contention
Iran's recent overtures signaling cooperation, though, likely stop short at the topics of Palestine and Syria. Iran is Syria's closest ally in the region.
In his speech Tuesday, Rouhani blasted what he described as the "structural violence" against Palestinians. While he never mentioned U.S. ally Israel by name, the intention of his comments was clear.
"Palestine is under occupation; the basic rights of the Palestinians are tragically violated, and they are deprived of the right of return and access to their homes, birthplace and homeland," he said. "Apartheid as a concept can hardly describe the crimes and the institutionalized aggression against the innocent Palestinian people."
On Syria, he said that the common objective of the international community regarding that country "should be a quick end of the killing of the innocent."
"We defend peace based on democracy and the ballot box everywhere, including in Syria, Bahrain and other countries in the region, and believe that there are no violent solutions to world crises," Rouhani said.
"While condemning any use of chemical weapons, we welcome Syria's acceptance of the Chemical Weapons Convention," he said, stressing that extremists' access to chemical weapons was the "greatest danger to the region."
Syria is under U.S.-led pressure to give up its chemical weapons arsenal in the aftermath of the August 21 attack on suburban Damascus that Washington and its allies blame on the al-Assad regime.
Obama said Tuesday that Syria's use of chemical weapons tested the relevance of the United Nations in the modern world, and he rejected contentions by the al-Assad regime and its main ally, Russia, that rebel forces were responsible for the attack.
"It is an insult to human reason -- and to the legitimacy of this institution -- to suggest that anyone other than the regime carried out this attack," Obama said.
Russia has blocked U.S. efforts to secure a strong Security Council resolution authorizing possible military force if Syria fails to comply with international regulations on turning over its chemical stockpiles. Obama argued Tuesday that such a resolution was vital.
"There must be a strong Security Council resolution to verify that the Assad regime is keeping its commitments, and there must be consequences if they fail to do so," he said. "If we cannot agree even on this, then it will show that the U.N. is incapable of enforcing the most basic of international laws. On the other hand, if we succeed, it will send a powerful message that the use of chemical weapons has no place in the 21st century, and that this body means what it says."
At the same time, Obama announced an additional $340 million in U.S. aid "to meet humanitarian needs in Syria and surrounding countries," referring to war refugees and other victims. The additional money increases the total U.S. commitment in humanitarian aid to $1.3 billion.
But Farah Atassi, activist with the opposition Syrian National Coalition, said Obama did not go far enough.
"The president did not address the core issue behind the Syrian crisis, which is holding the Assad regime accountable" -- including for its use of conventional weapons, she said in a CNN interview. More than 100,000 people have died in the conflict, the vast majority from conventional weapons, according to U.N. figures.
Obama is letting Syria's ally Russia lead, she said. "We don't trust the Russians."
"We want the U.S. to step forward and take the leadership right now to push for Assad to step down and allow for a transitional government from the opposition."
Emphasizing that the opposition has not asked for "boots on the ground," Atassi said U.S. military action should remain "on the table." And, she said, the rebels are looking to Washington for help empowering moderate elements of the opposition.
"We are not terrorists," she said.
Militant groups make up part of the Syrian opposition.
In his remarks to open the General Assembly on Tuesday, U.N. Secretary-General Ban Ki-moon called for the Syrian government to "fully and quickly" honor its obligations under the Chemical Weapons Convention, which calls for turning over control of its stockpiles.
Ban also appealed to all sides to stop supplying any weapons to all parties in the Syrian civil war while urging both the Syrian government and the opposition to respect international humanitarian law.
Russia and Iran, as well as Hezbollah in Lebanon, are providing military backing to the al-Assad regime while the United States and some European allies have started supplying light arms to rebel fighters.
Meanwhile, U.N. weapons inspectors will be back in Syria on Wednesday to assess at least six claims of chemical weapons use in Syria by the regime or rebels, a spokesman for Ban said Tuesday.
Al-Assad hinted at potential trouble for inspectors coming into Syria, saying other countries may order terrorists to attack them.
"Those militants might want to stop (the) experts' arrival. We know that those terrorists are under the control of some countries," he said in an interview Sunday with Chinese television. "And those countries may encourage the terrorists to stop experts from arrival, so that they could accuse the Syrian government for violating the agreement."
Despite al-Assad's veiled threat, positive progress has been made on the Syrian chemical weapons deal brokered by the United States and Russia in Geneva. Over the weekend, the United States said it was pleasantly surprised by the extent of Syria's initial declaration of its chemical weapons stockpile reported to the Organisation for the Prohibition of Chemical Weapons.
The United States is pushing for a U.N. Security Council resolution this week in New York to enforce the Geneva deal.
Brazil outrage over U.S. surveillance
Obama faced criticism Tuesday from Brazilian President Dilma Rousseff, who used her U.N. speech to call allegations of U.S. surveillance of her country "totally unacceptable."
Rousseff referred to classified leaks by former National Security Agency contractor Edward Snowden that made public how the U.S. government had access to phone and Internet records, including foreign information.
She said the U.S. surveillance intercepted private details of Brazilian citizens and businesses, along with "communications by Brazilian diplomatic representation offices, including the permanent mission of Brazil with the United Nations and even the very presidency of the republic of Brazil."
"Meddling in such a manner in the life and affairs of other countries is a breach of international law and such is an affrontment to the principles that should otherwise govern relations among countries, especially among friendly nations," she said, adding Brazil would propose U.N. action intended to prevent the manipulation of cyberspace as "a weapon of war."
Because of the surveillance controversy, Rousseff postponed a state visit to Washington that had been planned for next month. The White House said the postponement was a joint decision.
CNN's Dana Ford and Laura Bernardini contributed to this report.