WASHINGTON - US President Barack Obama warned Syrian President Bashar Assad on Friday that any use of chemical weapons in Syria's civil war would be a "game changer" but cautioned that intelligence assessments that such weapons had been deployed were still preliminary.Speaking a day after the White House said for the first time that Assad's government had likely used chemical weapons on a small scale, Obama talked tough while appealing for patience as he sought to fend off pressure at home and abroad for a swift US response.White House spokesman Jay Carney said on Friday the United States was continuing to study evidence and would not set a deadline for corroborating reports."We are continuing to work to build on the assessments made by the intelligence community, that the degrees of confidence here are varying, that this is not an airtight case," he said.In response to a question, Carney said Obama would consider a range of options including - but not exclusive to - military force, should it be determined that Syria has used chemical weapons."He retains all options to respond to that, all options," Carney said. "Often the discussion, when people mention all options are on the table, everyone just talks about military force. It's important to remember that there are options available to a commander in chief in a situation like this that include but are not exclusive to that option."Saying that confirmation was still needed to provide conclusive proof, Obama stopped short of declaring that Assad had crossed a "red line" he had warned earlier would unleash unspecified consequences, widely interpreted to include possible US military intervention."Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law," Obama told reporters at the White House as he met with Jordan's King Abdullah."That is going to be a game changer. We have to act prudently," he said. "We have to make these assessments deliberately. But I think all of us ... recognize how we cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations."Obama said the chemical weapons threat had added "increased urgency" in the Syrian crisis but cautioned that it would time to sort things out.In a shift from a White House assessment just days earlier, US officials said on Thursday the intelligence community believed with "varying degrees of confidence" that the chemical nerve agent sarin was used by Assad's forces against rebel fighters.The administration insisted, however, that Obama needed definitive proof before he would take action, making clear it he was mindful of the lessons of the start of the Iraq war more than a decade ago.Then, the George W. Bush administration used inaccurate intelligence to justify the invasion of Iraq in pursuit of nuclear, chemical and biological weapons that turned out not to exist.While some more hawkish US lawmakers have called for a US military response, several leading congressional voices called for a calmer approach on Friday after US Secretary of State John Kerry briefed them."This is not Libya," said Nancy Pelosi, the senior Democrat in the House of Representatives. "The Syrians have anti-aircraft capability that make going in there much more challenging."