Barack Obama has warned the Syrian regime that any use of chemical weapons on its own people would be "totally unacceptable" and would result in "consequences".
In strongly-worded remarks directed at President Bashar al-Assad, the US president said "the world is watching" and would act if the authorities escalated the conflict through the use of chemical warfare.
Intelligence reports suggest sensitive chemical weapons parts have been moved around the country in recent days. This has raised fears that the regime is considering unleashing chemical weapons = on opposition forces.
Obama told a gathering of nuclear proliferation experts in Washington: "The use of chemical weapons is, and would be, totally unacceptable and if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable."
Obama did not outline what consequences would be taken.
His words echoed a similar warning to Damascus from the US secretary of state, Hillary Clinton: "I am not going to telegraph any specifics what we do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people, but suffice to say we are certainly planning to take action if that eventuality were to occur."
CNN reported that Syrian forces had started to combine chemicals to make deadly sarin gas which it could deploy against anti-government fighters. The network cited an unnamed US official as the source of its report.
Damascus has denied the suggestion that it would resort to chemical warfare. "Syria has stressed repeatedly that it will not use these types of weapons, if they were available, under any circumstances against its people," the foreign ministry said on Monday.
With little indication of an end to the conflict in Syria and signs that Assad's regime is becoming more desperate in the face of persistent opposition forces, the United Nations is preparing to evacuate all non-essential staff from Syria.
Those who remain in the country will be on standby to move to places of safety, said the UN, citing the "prevailing security situation" amid growing fears in Washington that the beleaguered regime is considering using chemical weapons. The European Union also announced it was cutting its activities in the country.
On a fast-moving day of diplomatic and military action, the Syrian government's foreign ministry spokesman, Jihad Makdissi, was reported to have defected.
The UN's under-secretary for safety and security, Gregory Starr, said it had cancelled all missions to Syria from abroad and suspended its activities inside the country. The decision marks the final step before a full-scale evacuation, a move that has not been ordered at any point during Syria's steady descent into chaos over the past 20 months.
Up to 25 of about 100 foreign staff may leave this week, the UN said, adding that more armoured vehicles were needed after attacks in recent weeks on humanitarian aid convoys and the hijacking of goods or vehicles. Some convoys were caught in crossfire between government and rebel forces, it said, including an incident near the airport in which two staff were injured.
The UN deploys more than 1,000 national and international staff in Syria, but its Office for the Co-ordination of Humanitarian Affairs said movement and communications had become more difficult due to intensified fighting near the capital and a 48-hour internet blackout last week.
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