WASHINGTON: President Barack Obama on Monday warned Syrian President Bashar al-Assad that the use or deployment of chemical or biological weapons in his country's conflict would be a "red line" for the United States as it views Damascus's suppression of the uprising.

Noting that he had not "at this point" ordered US military engagement in Syria, Obama told reporters during an unscheduled appearance in the White House briefing room that the issue of chemical and biological weapons was of high concern to the both the United States and its close ally, Israel.

"A red line for us is (if) we see a whole bunch of chemical weapons moving around, or being utilized. That would change my calculus," Obama said.