President Obama warned Syrian President Bashar al-Assad on Monday that "there will be consequences" if Assad unleashes chemical weapons against anti-government forces, an increasingly likely prospect that could prompt U.S. intervention in the nation's deadly civil war.

Obama's warning came after intelligence officials detected movement at several Syrian chemical weapons sites in recent days, though the president and senior administration officials cautiously avoided any commitment to direct intervention in the war-torn nation.

"The world is watching," Obama warned in a speech at the National Defense University. "The use of chemical weapons is and would be totally unacceptable. ... [Y]ou will be held accountable."

Both the White House and Secretary of State Hillary Clinton said the use of chemical weapons by Syria represented a "red line," but they declined to specify how the U.S. would respond. Syria reacted by downplaying international concerns, saying it would not use chemical weapons against its own people.

Deploying such weapons would likely draw widespread international condemnation and heighten concerns that Syria would launch attacks on Israel, also leaving Obama with little choice but to intervene militarily, defense analysts said.

"Obviously, chemical weapons would change the White House's calculation," said Lawrence Korb, assistant defense secretary under President Reagan. "Then you're talking about weapons of mass destruction. The public understands that -- and it's not like the U.S. would be sending thousands and thousands of troops into Syria."

The White House has been criticized for choosing to get involved in Libya but not Syria, where 40,000 people have been killed since Assad began cracking down on opposition forces 20 months ago. Intelligence officials estimate Syria has several hundred ballistic surface-to-surface missiles that could carry chemical warheads.

"The risks of intervening are very high, but the costs of not doing anything are growing over time," said Christopher Chivvis, of the nonpartisan RAND Corp. "I see the debate shifting, especially of late."

Assad's government in July said that while it wouldn't use chemical weapons against its own citizens, it is ready to launch them against any outside force that intervenes in Syria's civil war.

With intelligence sources showing activity around Syrian weapons sites in recent days, the United Nations announced Monday that it is withdrawing "all nonessential international staff" from the country.

The rebel forces in Syria have made a series of advances in recent weeks, which has only increased international speculation over whether a desperate Assad could resort to gassing his own people to hold onto power.

"We once again issue a very strong warning to the Assad regime that their behavior is reprehensible. Their actions against their own people have been tragic," Clinton said. "But there is no doubt that there's a line between even the horrors that they've already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."