U.S. President Barack Obama says he will not stand by idly if Syrian President Bashar al-Assad’s government uses biological weapons.

During a news conference at the White House on Monday, Obama said the U.S. has been very clear to the Assad regime that "a red line for” the U.S. is when Washington starts seeing chemical weapons moving around or being utilized.

The U.S. president said at this point he has not ordered military intervention in the Syrian conflict, but added he would change “his equation” if Syria’s government shows signs of using such weapons.

Obama's remarks came amid reports that the Syrian government transported biological weapons to key areas of confrontation with antigovernment rebels.