WASHINGTON (AP) — The top U.S. military commander in Europe said Tuesday that several NATO countries are working on contingency plans for possible military action to end the two-year civil war in Syria as President Bashar Assad’s regime accused U.S.-backed Syrian rebels of using chemical weapons.

The Obama administration rejected the Assad claim as a sign of desperation by a besieged government intent on drawing attention from its war atrocities — some 70,000 dead, more than 1 million refugees and 2.5 million people internally displaced. A U.S. official said there was no evidence that either Assad forces or the opposition had used chemical weapons in an attack in northern Syria.