This story was originally published Dec. 4, 2012:

WASHINGTON - President Obama warned the embattled leader of war-torn Syria on Monday not to use chemical weapons against rebels fighting to topple his regime, as the United States voiced rising concern that he may be preparing to do so and consulted with regional allies on a range of responses.

Obama's admonition to Syrian President Bashar al-Assad was his most strident since he drew a "red line" in August that the movement of chemical or biological weapons out of their secure sites, or their use, would bring U.S. military intervention in the country's vicious civil war.

His latest comments came shortly after the White House spoke of growing worries that Assad could be preparing to use chemical weapons against rebel forces that have been gaining ground in northern and eastern Syria, and have staged attacks in the capital, Damascus, which compelled the closure of the international airport.

Administration officials, however, offered no public evidence justifying their heightened fears, citing classified intelligence.

"This is not just analytic conjecture. It is firm information," insisted a senior U.S. official, who requested anonymity because of the matter's sensitivity. "We're wary about indications we've seen that the Syrian regime may be in the midst of preparing chemical weapons for possible use."

Obama aimed his warning directly at Assad and his top lieutenants in a speech to a nonproliferation symposium at the National Defense University.

The United States has "worked to keep weapons from spreading, whether it was nuclear material in Libya or, now, chemical weapons in Syria," the president said. "And today, I want to make it absolutely clear to Assad and those under his command: The world is watching. 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," he said, without elaborating.

Obama's warning echoed comments issued earlier in the day by Secretary of State Hillary Clinton during a visit to Prague, the capital of the Czech Republic, and by spokesmen for the White House, the Pentagon, and the State Department.

"As the opposition makes strategic advances, and grows in strength, the Assad regime has been unable to halt the opposition's progress through conventional means," said White House spokesman Jay Carney. "And we are concerned that in an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people."

Carney indicated that the United States is preparing military options, saying that "contingency planning of all kinds is the responsible thing to do." Washington also is consulting with its regional allies and Syrian opposition groups, he said.

Syria is believed to maintain stockpiles comprising unknown quantities of biological weapons and hundreds of tons of chemical weapons.

Speaking in Prague, Clinton declined to discuss the U.S. response should Assad use chemical weapons. "This is a red line for the United States," she said. "I am not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people. But suffice it to say, we are certainly planning to take action if that eventuality were to occur."

The United States is supporting a request by fellow NATO member Turkey for the alliance to deploy U.S.-made Patriot antimissile batteries to defend its border with Syria against attack by Syrian missiles tipped with chemical warheads. Foreign ministers of the 28-member pact are expected to approve the deployment at a meeting this week in Brussels.