WASHINGTON - US President Barack Obama bluntly warned Syrian President Bashar Assad on Monday not to cross a "red line" by using chemical or biological weapons in his country's bloody conflict and suggested that such action would prompt the United States to consider a military response.Pointing out that he had refrained "at this point" from ordering US military engagement in Syria, Obama said that there would be "enormous consequences" if Assad failed to safeguard his weapons of mass destruction.Syria last month acknowledged for the first time that it had chemical and biological weapons and said it could use them if foreign countries intervene -- a threat that drew strong warnings from Washington and its allies.Western countries and Israel have expressed fears chemical weapons could fall into the hands of militant groups as Assad's authority erodes.Israel has said that if Syrian-backed Hezbollah guerrillas used the situation to take control of the weapons, it would "act immediately and with utmost force.""We're monitoring that situation very carefully. We have put together a range of contingency plans," Obama said when asked whether he envisioned the possibility of using US forces at least to safeguard Syria's chemical arsenal.The Global Security website, which collects published intelligence reports and other data, says there are four suspected chemical weapons sites in Syria: north of Damascus, near Homs, in Hama and near the Mediterranean port of Latakia. Weapons it produces include the nerve agents VX, sarin and tabun, it said, without citing its sources.Obama also used the opportunity to renew his call for Assad to step down."The international community has sent a clear message that rather than drag his country into civil war, he should move in the direction of a political transition," Obama said. "But at this point, the likelihood of a soft landing seems pretty distant."Obama said the United States had already provided $82 million in humanitarian assistance for Syria refugees and "we'll probably end up doing a little bit more" to keep the situation from destabilizing Syria's neighbors.It was Obama's strongest language to date on the issue, and he warned Syria not only against using its unconventional weapons, but against moving them in a threatening fashion."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 utilized," Obama said. "That would change my calculus.""We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," Obama told an impromptu White House news conference. He acknowledged he was not "absolutely confident" the stockpile was secure.Obama said the issue was of concern not only to Washington but also to its close allies in the region, including Israel.Seeking re-election in November, Obama has been reluctant to get the United States involved in another war in the Middle East, even refusing to arm rebels fighting a 17-month-old uprising against Assad.