"We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," Obama told reporters at the White House. "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.

Obama added that U.S. officials are monitoring the situation "very carefully" and have put together a number of contingency plans.

"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."

The Syrian regime "probably has the largest and most advanced chemical warfare program in the Arab world," Michael Eisenstadt, director of the military and security studies program at the Washington Institute for Near East Policy, told CNN last month.

It includes "thousands of tube and rocket artillery rounds filled with mustard-type blister agents, thousands of bombs filled with the nerve agents sarin and possibly VX, and binary-type and cluster CW warheads filled with nerve agents for all its major missile systems.

Syria is not a party to the Organization for the Prohibition of Chemical Weapons. That's the body of the Chemical Weapons Convention dedicated to eliminating the weapons.