US warns Syria on weapons

"A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilised," Mr Obama told reporters. "That would change my calculus."

Mr Obama said the US has "put together a range of contingency plans" for how to deal with Syrian President Bashar al-Assad's regime, its violence towards Syrians and other forces and has made clear to regional players that there would be "enormous consequences" for any development involving Syria's chemical weapons arsenal.

While the US has urged Dr Assad to step down, Mr Obama said that "at this point the likelihood of a soft landing seems pretty distant".

Syria's Foreign Ministry spokesman said last month that chemical weapons are secure and won't be used against the country's opposition. The spokesman, Jihad Makdissi, indicated that assurance may not apply to foreign intervention.

"All the stocks of these weapons that the Syrian Arab Republic possesses are monitored and guarded by the Syrian Army," he said at a July 23 Damascus press conference shown on state-run television. "These weapons are meant to be used only and strictly in the event of external aggression."

The Foreign Ministry later issued a statement saying Mr Makdissi was responding to a "false premise" and was not confirming for the first time that Syria had chemical weapons.

A senior Obama administration official emphasised that the President's warning was aimed at large-scale transfers of weapons that would make them vulnerable to capture by radical forces, not movements by the government intended to secure the arsenal.

By hinting that the US might participate in locating and neutralising the weapons, Mr Obama was clearly trying to forestall the possibility of an Israeli move into Syria and the reaction it might provoke.

"The problem is that the material is so dispersed," said an expert who has been consulted by the administration.

While the intelligence about the stockpiles is sketchy, US estimates indicate there could be as many as two dozen sites around the country.

The search for Syria's unconventional weapons is yet another example of how much more complicated the situation in Syria is for the US than was Libya a year ago. In Libya, the weapons of greatest concern were shoulder-fired anti-aircraft munitions. They were tracked down by the US and Britain, largely using outside contractors and covert operatives. But chemical and biological weapons are harder to track.

In Syria, Dr Assad's forces stepped up attacks in and around the south-western city of Deraa on Monday, with activists reporting raids, executions of suspected opposition figures and intensified shelling.

The Japanese Foreign Ministry said that Japanese journalist Mika Yamamoto had been killed during fighting in Aleppo.

Her body has been transferred to a hospital in Kilis province in Turkey, it said.