Secretary of State Condoleezza Rice said Pakistan must show "resolve and urgency" as she called Wednesday for international cooperation in the investigation into the Mumbai attacks.
Rice arrived in New Delhi as part of a U.S. effort to ease tensions in the region after a three-day terrorist attack killed 171 people in India's financial capital.
"I have said that Pakistan needs to act with resolve and urgency, and cooperate fully and transparently," Rice said during a press conference. "I know too this is a time when cooperation of all parties who have any information is really required."
Indian and U.S. officials have pointed the finger at Pakistani-based groups in the attacks.
Rice said it was too early to say who was responsible for the attack, but: "Whether there is a direct al-Qaida hand or not, this is clearly the kind of terror in which al-Qaida participates."
Rice, America's top diplomat, was to meet with Indian Prime Minister Manmohan Singh and other top officials.
The Indian government, already facing accusations of security and intelligence failures, has demanded that Pakistan take action against those responsible and asked that 20 suspected terrorists believed living in the country be handed over.
However, Pakistani President Asif Zardari said any of the 20 suspects wanted by India would be tried in Pakistan if there is evidence of wrongdoing.
Zardari said he would "look into all the possibility of any proof" about the suspects sought by India and insisted they would be dealt with under Pakistani law.
"At the moment, these are just names of individuals - no proof and no investigation," he said Tuesday in an interview with CNN's Larry King. "If we had the proof, we would try them in our courts and we would try them in our land and we would sentence them."
India has stepped up the pressure on its neighbor after interrogating the only surviving attacker, who told police that he and the other nine gunmen had trained for months in camps in Pakistan operated by the banned Pakistani militant group Lashkar-e-Taiba.
U.S. National Intelligence Director Mike McConnell said Tuesday the same group that carried out last week's attack is believed to be behind the Mumbai train bombings that killed more than 200 people two years ago.
While he didn't identify the group, the Indian government has attributed the 2006 attack to Lashkar and the Students Islamic Movement of India.
Authorities said Tuesday that ex-Pakistani army officers trained the gunmen behind the attacks - some for up to 18 months - and the group set out by boat from the Pakistani port of Karachi.
The revelations came as a senior Bush administration official said India had received a warning from the United States that militants were plotting a waterborne assault on Mumbai. The official, who spoke on condition of anonymity because of the sensitive nature of intelligence information, would not elaborate on the timing or details of the U.S. warning.
Last week's attacks against hotels, a restaurant and other sites across this sprawling city killed 171 people, including 26 foreigners, officials said Wednesday. The death toll was revised down from 172 after authorities realized they had counted a victim twice.
"More bodies being found is ruled out," Maharashtra state government spokesman Bhushan Gagrani said.
The rampage has exposed weakness in India's security and intelligence agencies, which apparently failed to act on multiple warnings ahead of the Mumbai attacks - "a systemic failure," said Indian navy chief Sureesh Mehta.
India's foreign intelligence agency also had warnings as recently as September that Pakistan-based terrorists were plotting attacks on Mumbai, according to a government intelligence official familiar with the matter.
The information, intercepted from telephone conversations apparently coming out of Pakistan, indicated that hotels might be targeted but did not specify which ones, said the official, who spoke on condition of anonymity because he was not authorized to talk publicly about the details.
At the Taj Mahal Hotel, where the siege finally ended Saturday morning, authorities tightened security in the weeks before the attacks after being warned of a possible threat.
At the Oberoi hotel, the second five-start hotel the gunmen seized, the shopping arcade opened Wednesday for the first time since the attacks.
Ajmal Qasab told police his group trained for about six months in Lashkar camps in Pakistan, learning close-combat techniques, hostage-taking, handling of explosives, satellite navigation.
The training was "meticulous and rigorous," said a security official who spoke on the customary condition of anonymity.
The official said the gunmen sailed from Karachi in a Lashkar vessel that brought them to the waters near an Indian vessel they hijacked, the MV Kuber.
They killed three crew members and dumped their bodies in high seas, but kept the captain alive so that he could guide them into Mumbai.
The captain was killed some three nautical miles off Mumbai's coast, the official said.
Police were questioning the owner of the MV Kuber, from which investigators recovered a global positioning system that belonged to the attackers.
Associated Press writers Ravi Nessman and Ramola Talwar Badam in Mumbai, Ashok Sharma and Ane Gearan in New Delhi, Asif Shahzad in Islamabad, Pakistan, and Jennifer Loven in Washington contributed to this report.
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