WASHINGTON – President Obama on Monday warned Syria’s Bashar al-Assad not to use or distribute its stock of chemical weapons, as U.S. officials saw evidence that the country’s embattled regime may be preparing to use such weapons to repel advances by rebel fighters.

“Today I want to make it absolutely clear to Assad and those under his command: the world is watching,” Obama said in a speech at the National Defense University in Washington. “The use of chemical weapons is and would be totally unacceptable, and if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable.”

U.S. reconnaissance satellites last week detected increased activity at several chemical weapons depots in Syria, including movement of some weapons into position where President Assad’s forces could more quickly deliver them by aircraft or artillery, two U.S. officials said in interviews.

Intelligence sources also picked up orders to at least one Syrian military base to begin combining the two precursor chemical components of Sarin nerve gas to make it ready to use, the officials said, speaking on condition of anonymity to discuss intelligence details. Syrian authorities have stored the components, which produce a deadly nerve agent when mixed, in secure separate facilities, according to the officials.

The Syrian foreign ministry denied that the regime planned to use chemical weapons against it own people.

The 20-month uprising against Assad’s minority Alawite ruling party — which the opposition Syrian Observatory for Human Rights said has killed more than 30,000 people — appears to be at or near a decisive moment, according to the U.S. officials. Regime leaders may fear that they may soon be forced to decide whether to flee into exile, fight to the death or delay defeat by using chemical weapons, the officials said.

The fighting has drawn close to Damascus, the capital, and rebels fighting to overthrow the Assad regime have gained the ability to shoot down Syrian warplanes and helicopters with shoulder-fired anti-aircraft missiles supplied by Persian Gulf states, the U.S. officials said.

The Syrian government Monday sent fighter jets to bomb suburbs around Damascus to keep rebel fighters from the center of the city, the Voice of America reported, citing the Britain-based Syrian Observatory.

The United States has been working with Jordan and Turkey in an effort to monitor chemical weapons sites “and try to determine how best to respond to any concerns in that area,” Defense Secretary Leon Panetta said in October.

“A group of our forces” are in Jordan to help build a headquarters and assist with “possible consequences of what’s happening in Syria,” Panetta said at an Oct. 10 news conference in Brussels.

The United States will take action if it sees evidence that the Syrian government is using chemical weapons, Secretary of State Hillary Clinton said Monday. “This is a red line for the United States,” she said in Prague, declining to specify what specific action the United States might take.

In Washington, State Department spokesman Mark Toner said the Syrian regime would cross the U.S. “red line” if it provides chemical weapons to others as well as if it uses such weapons itself.

White House spokesman Jay Carney said concern “has increased” that the Assad regime might turn to its chemical weapons stockpiles as it fails to stem the upheaval by conventional means.

The United States last publicly warned Syria about chemical weapons in July when Obama said the regime “will be held accountable” if the weapons were unleashed, after the Syrian Foreign Ministry said the nation’s stockpiles were secure.

Syria has repeatedly said that it won’t use any chemical weapons, if they exist, against its people no matter the circumstances, an official source at the Foreign and Expatriates Ministry said Monday, according to the website of Syrian state television. The Syrian source, who wasn’t identified, said the United States has a record of fabricating such issues as a pretext for war, citing the invasion of Iraq, according to the report.

Citing the worsening security situation in Syria, the United Nations on Monday ordered the pullout of all of its non-essential staff, according to spokesman Martin Nesirky.

Turkey has asked NATO to deploy Patriot missile batteries on its soil for the first time in about a decade amid fears that Syria may launch a punitive attack, which could include the use of chemical weapons, against the country for backing Syrian rebels.

Israeli Prime Minister Benjamin Netanyahu has said he is concerned that Hezbollah or other groups could gain control of Syria’s chemical weapons if Assad’s regime collapses. Netanyahu has said he wouldn’t rule out Israeli action in such a case.