President Barack Obama warned Monday that he would consider military force if Syria's Bashar Assad either tries to relocate his regime's arsenal of chemical and biological weapons or attempts to use them on his own people in a bid to crush an uprising that could topple him from power.

"I have, at this point, not ordered military engagement in the situation," Obama told reporters during a surprise question-and-answer session in the White House briefing room.

"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation," he said.

Syria said in late July that it possessed chemical weapons and could use them to fend off any "external aggression" but not to crush rebels against Assad's rule.

"We're monitoring that situation very carefully. We have put together a range of contingency plans," Obama said. "That's an issue that doesn't just concern Syria. It concerns our close allies in the region, including Israel. It concerns us. We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people."

"We have communicated in no uncertain terms with every player in the region that that's a red line for us and that there would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons. That would change my calculations significantly."