President Barack Obama on Monday warned Syrian President Bashar Assad that the use or deployment of chemical or biological weapons in his country's conflict would be a "red line" for the United States as it views Damascus's suppression of the uprising.

Noting that he had not "at this point" ordered U.S. military engagement in Syria, Obama told reporters during an unscheduled appearance in the White House briefing room that the issue of chemical and biological weapons was of high concern to the both the United States and its close ally, Israel.

"We have been very clear to the Assad regime but also to other players on the ground that a red line for us is [if] we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation," Obama said.

Obama said the use of such weapons of mass destruction, which Syria has, would widen the conflict considerably.

"The point about chemical and biological weapons is critical. That's an issue that doesn't just concern Syria. It concerns our close allies in the region, including Israel," he said.

"I have indicated repeatedly that President al-Assad has lost legitimacy, [and] that he needs to step down. So far, he hasn't gotten the message and instead has doubled down in violence on his own people," he said. "At this point the likelihood of a soft landing seems pretty distant."

U.S. and diplomatic officials have recently said that the United States and its allies are discussing a worst-case scenario that could require tens of thousands of ground troops to go into Syria to secure chemical and biological weapons sites following the fall of President Bashar Assad's government.

A U.S. official, speaking on condition of anonymity to explain the sensitive discussions, said the United States still had no plans to put boots on the ground in Syria. President Barack Obama's administration has, in fact, so far refused to provide lethal support to the rebels fighting to oust Assad's regime and the Pentagon has played down the possibility of implementing a no-fly zone anytime soon.

Meanwhile, State Department spokeswoman Victoria Nuland carefully answered questions on the statement by newly appointed UN and the Arab League envoy to Syria, Lakhdar Brahimi, that it's too early to call for Assad to step down.

"Our position is well-known," she said. "We don't think there is going to be peace in Syria until Assad steps down and the bloodshed ends. We will be clear with Special Envoy Brahimi about our views on this. But I think it's fair to let him get his feet under him and get started on the job and have the consultations that he needs to have before he draws his own conclusion."