President Obama said Monday the U.S. would reconsider its opposition to military involvement in the Syrian civil war if President Bashar Assad’s beleaguered regime deploys or uses chemical or biological weapons. He called such action a “red line” for the United States.

Speaking to reporters at the White House, Obama said the use of such weapons of mass destruction would considerably widen a conflict that has already dragged on for a year-and-a-half and killed some 20,000 people, according to activists. Syria possesses extensive chemical and biological weapons stockpiles and has threatened to use them if the country comes under foreign attack.