US President Barack Obama said on Monday that if Syria were to move or use its chemical weapons it would be a "red line" that would change his perspective on how to respond to the conflict.

Obama said he had not ordered US military intervention "at this point," but warned that the United States was "monitoring the situation very carefully, and we have put together a range of contingency plans.

"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," he told reporters at a White House briefing.

Responding to a question about the Syrian arsenal, Obama said: "I have at this point not ordered military engagement in the situation, but that point that you made about chemical and biological weapons is critical.

"It is not just an issue that concerns Syria, but 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.

Bashar al-Assad's beleaguered regime is reported to have access to weapons containing nerve gas or germ warfare agents, and Obama said the regime has been warned that any recourse to such arms would be unacceptable.

"And we have communicated in no uncertain terms with every player in the region, that this is a red line for us and that there would be enormous consequences," he said.

"If we start seeing movement on the chemical weapons front or the use of chemical weapons that would change my calculations significantly."