US 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 US 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 US 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.

Call to quit

Calling again for Assad to step aside to allow a democratic transition but conceding that prospects for a "soft landing" to the conflict were dim, Obama said the weapons worried not only Washington but also its allies in the region, including Israel.

Obama has been reluctant to get the United States involved in another war in the Middle East, and refuses to arm rebels fighting a 17-month uprising against Assad, partly out of concern that some of those fighting against the Iranian-backed president are Islamist radicals equally hostile to the West.

However, Obama said, Assad should quit: "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."

When asked whether he envisioned the possibility of using US forces at least to safeguard Syria's chemical arsenal, he said: "We're monitoring that situation very carefully. We have put together a range of contingency plans."

The US-based Global Security website says there are four suspected chemical weapons sites in Syria producing the nerve agents VX, sarin and tabun. It does not cite its sources. – Reuters