WASHINGTON — President Obama warned Syria on Monday not to use chemical weapons against its own people, vowing to hold accountable anyone who did, even as American intelligence officials picked up signs that such arms might be deployed in the fighting there.

The White House said it had an “increased concern” that the government of President Bashar al-Assad was preparing to use such weapons, effectively confirming earlier reports of activity at chemical weapons sites. The administration said it would take action if they were used, suggesting even the possibility of military force.

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

Neither the president nor his aides would specify how the United States would hold Syrians accountable, but the White House confirmed that contingency plans had been drawn for direct action. The president’s statement amplified similar warnings issued by Secretary of State Hillary Rodham Clinton earlier in the day in Prague, the capital of the Czech Republic, which represents the interests of the United States in Damascus now that the American Embassy there has been closed.