U.S. forces could move against Syrian leader Bashar al-Assad, President Barack Obama warned, notably if he deploys his chemical weapons against rebels trying to overthrow him.In some of his strongest language yet on Syria, on a day when U.N. observers pulled out after a fruitless bid for peace and Assad’s forces mounted new attacks, the U.S. leader said Assad faced "enormous consequences” if he crossed a "red line” of even moving unconventional weapons in a threatening manner.Seeking re-election in November, Obama noted that he had refrained "at this point” from ordering U.S. military engagement in Syria. But when he was asked at a White House news conference whether he might deploy forces, for example to secure Syrian chemical and biological weapons, he said his view could change."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 utilised,” Obama said. "That would change my calculus.”Faced with a complex and explosive conflict at the heart of the Middle East, and with resolute support for Assad from Iran and from Russia and China at the United Nations, Washington and its Western allies have shown little appetite for more than hands-off help for the rebels, in contrast to their attacks on Libya’s Muammar Gaddafi last year. Obama’s comments, however, raised the prospect of some change, under certain conditions.Syria last month acknowledged for the first time that it had chemical and biological weapons and said it could use them if foreign countries intervened. The threat drew strong warnings from Washington and its allies, although it is not clear how the Syrian armed forces might use such weapons in urban warfare."We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people,” Obama told the impromptu news conference on Monday. He acknowledged he was not "absolutely confident” the stockpile was secure.