WASHINGTON/ DAMASCUS (CNN) -- Syrian President Bashar al-Assad should heed U.S. warnings to neither use nor move chemical or biological weapons, lest he risk crossing a "red line" and provoke a U.S. military response, President Barack Obama said Monday.
"We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," Obama told reporters at the White House. "We have been very clear to the Assad regime -- but also to other players on the ground -- that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized.
"That would change my calculus; that would change my equation."
Obama added that U.S. officials are monitoring the situation "very carefully" and have put together a number of contingency plans.
"We have communicated in no uncertain terms with every player in the region that that's a red line for us and that there would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons."
Obama's remarks appeared to ratchet up his stance on the matter. Last month, talking about Syrian forces, he told a VFW convention, "They will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons."
The Syrian regime "probably has the largest and most advanced chemical warfare program in the Arab world," Michael Eisenstadt, senior fellow and director of the military and security studies program at the Washington Institute for Near East Policy, told CNN last month.
It includes "thousands of tube and rocket artillery rounds filled with mustard-type blister agents, thousands of bombs filled with the nerve agents sarin and possibly VX , and binary-type and cluster CW warheads filled with nerve agents for all its major missile systems.
"Its CW infrastructure is believed to include several production facilities and numerous storage sites, mostly dispersed throughout the western half of the country," Eisenstadt said.
Syria is thought to have a biological warfare research and development program, but is not known to have offensive biological warfare agents, Eisenstadt said. The nation is not a party to the Organization for the Prohibition of Chemical Weapons. That's the body of the Chemical Weapons Convention dedicated to eliminating the weapons.
Israeli President Shimon Peres has said his country too is preparing contingency plans to attack Syria's chemical weapons arsenal if Israel is directly threatened. In September 2007, Israeli jets bombed a building in Syria that the U.N. nuclear agency eventually concluded was "very likely a nuclear reactor."
Obama's warning on Monday came as Muslims in Syria celebrated the second of three days of feasting to honor the end of the holy month of Ramadan, the holiday's message of amity and tolerance contrasted sharply with reality.
At least 150 people were killed Monday, including two children and their mother in Daraa, the Local Coordination Committees of Syria opposition group said.
Among them were 10 bodies discovered splayed on the ground in the Damascus suburb of Qaboun. Video posted by activists showed at least one of the victims lying on his back, blindfolded, his arms stretched above his head. Others showed victims with bruised flesh and dark red splotches on their clothes. CNN cannot authenticate the video.
Another video, said to have been shot Sunday in the town of Talbeesa, north of Homs, showed children being pushed on swings. The swings are attached to the gun of a tank on which adults are standing, chanting, "Victory for the rebels!"
In a third video, showing the same tank, a man says to the camera, "We will enter Homs with this tank and liberate all of Syria, God willing!"
Daraa province, where the conflict began in March 2011 as peaceful anti-government demonstrations, has suffered heightened violence in recent days.
It is also the home province of Vice President Farouq al-Sharaa, who rebels say has defected from the regime.
On Sunday, the charred bodies of 20 people were discovered in the Daraa city of Hirak, and 31 other people were killed in the province, the LCC said. Government forces executed 14 people in Daraa hours after they were detained, the opposition network said.
A Japanese journalist was wounded in artillery shelling in the Aleppo neighborhood of Suleiman, the London-based opposition Syrian Observatory for Human Rights reported.
Hirak took another beating Monday, when mortars and rockets rained down on the city, the LCC said. The reports came a day after the mandate of the U.N. observer mission in Syria ended, with the country no closer to a viable cease-fire than it was when monitors arrived four months ago.
The U.N. monitoring mission arrived in Syria in April, on the heels of a cease-fire that never took hold.
Over the past month, opposition activists have reported an average of more than 100 deaths per day in the fighting, which has become a civil war. On Sunday, when much of the country was celebrating the first day of Eid al-Fitr, at least 170 people were killed in cities nationwide, the LCC said.
Eid marks the end of Ramadan, Islam's holiest month, when Muslims fast from sunrise to sunset.
State-run TV aired footage Sunday of Eid prayers in Damascus, attended by President al-Assad and other Syrian leaders. Al-Sharaa, the vice president, was absent from the service.
Louai Miqdad, a spokesman for the rebel Free Syrian Army, said Saturday that al-Sharaa had defected, leaving Damascus more than a week ago and heading to his home province of Daraa to try to ensure the safety of relatives.
Miqdad said rebel fighters were trying to help get al-Sharaa out of the country and into neighboring Jordan. The official Syrian Arab News Agency said Saturday that the vice president's office had issued a statement saying he "has never at any moment thought of leaving the homeland to whatever direction."
It added that al-Sharaa has been working "to achieve a national reconciliation that maintains the country's territorial integrity, regional safety and national independence far from any foreign military intervention."
If al-Sharaa did defect, it would mark the latest in a stream of recent defections by Syrian officials, including Republican Guard Brig. Gen. Manaf Tlas and Prime Minister Riyad Hijab. Like al-Sharaa, the men are Sunni Muslims who held top posts in a government dominated by the country's Alawite minority.
Since the Syrian crisis began 17 months ago, more than 20,000 people -- most of them civilians -- have been killed, opposition activists say.