WASHINGTON -- President Obama on Monday threatened U.S. military action against Syria if there was evidence that the government of President Bashar Assad was moving its chemical or biological weapons.
It was Mr. Obama's most direct warning of American intervention in Syria, where the military is fighting an 18-month rebellion.
"We cannot have a situation in which chemical or biological weapons are falling into the hands of the wrong people," Mr. Obama said in an impromptu appearance at the White House. "We have put together a range of contingency plans. We have communicated in no uncertain terms with every player in the region that that's a red line for us."
The President said he was deeply troubled by the prospect that Assad's forces might move or even use the weapons in its increasingly harsh effort to crush the uprising,
Pointing out that he had refrained "at this point" from ordering U.S. military engagement in Syria's bloody conflict, Mr. Obama said there would be "enormous consequences" if Assad failed to safeguard his weapons of mass destruction.
The language was Mr. Obama's most explicit to date on the prospects for military intervention, and he warned Syria not only against using its unconventional weapons but against moving them in a threatening fashion.
He acknowledged he was not "absolutely confident" the stockpile was secure.
Mr. Obama said the issue was of concern to Washington and to its close allies in the region, including Israel.
The President's announcement came as U.N. observers ended their four-month mission in Damascus.
The United States and its allies are discussing a worst-case scenario that could require tens of thousands of ground troops to go into Syria to secure chemical and biological weapons sites following the fall of Assad's government, U.S. and diplomatic officials told Reuters last week.
These secret discussions assume that all of Assad's security forces disintegrate, leaving weapons sites in Syria vulnerable to pillaging.
The scenario also assumes these sites could not be secured or destroyed solely through aerial bombings, given health and environmental risks.
But a U.S. official, speaking on condition of anonymity, said the United States had no plans to put troops in Syria.
Syria last month acknowledged that it had chemical and biological weapons and said it could use them if foreign countries intervene.
Israel has said that if Syrian-backed Hezbollah guerrillas used the situation to take control of the weapons, it would "act immediately and with utmost force."
"We're monitoring that situation very carefully," Mr. Obama said when asked whether he envisioned the possibility of using U.S. forces at least to safeguard Syria's chemical arsenal.
The Global Security Web site, which collects published intelligence reports and other data, says there are four suspected chemical weapons sites in Syria: north of Damascus, near Homs, in Hama, and near the Mediterranean port of Latakia.
Syria is believed to have accumulated huge supplies of mustard gas, sarin nerve agent, and cyanide.
Mr. Obama also renewed his call on Monday for Assad to step down.
"The international community has sent a clear message that rather than drag his country into civil war, he should move in the direction of a political transition," Mr. Obama said.
In Syria, government forces pummeled the battered city of Aleppo with airstrikes and tanks and shelled parts of Damascus on Monday, killing at least 100 people during a major Muslim holiday, rights groups and activists said.
The violence escalated dramatically after a one-day lull Sunday, the start of the three-day Eid al-Fitr holiday which marks the end of the holy month of Ramadan.
Most of the deaths Monday were a result of tank and mortar shelling, and clashes in the Damascus suburbs of Daraya and Moadamiyeh, where activists reported the regime used helicopter gunships. The Syrian Observatory for Human Rights and others said up to 31 people were killed.
An activist, El-Said Mohammed, said about 30 troops along with a tank defected to the rebels' side in Moadamiyeh on Sunday, which may have been the reason for Monday's shelling.
Mohammed spoke by Skype from the Damascus area. His information could not be verified.
Both the Observatory, which relies on a network of activists on the ground, and a group called the Local Coordination Committees, reported at least 100 civilian deaths across the country.
Anti-regime activists say some 20,000 people have been killed since the revolt began in March, 2011.
The U.N.'s new envoy to Syria acknowledged Sunday that he had no concrete ideas to end the conflict and that his mission would be difficult without a unified position by the U.N. Security Council.
"The problem is not what I can do differently, it is how others are going to behave differently," Lakhdar Brahimi told Associated Press at his Paris home.