US warning to Assad amid 'credible' claims on Syria's chemical weapons But with Turkey already warning that the Syrian regime might use ballistic missiles, perhaps with chemical warheads, as a last resort, Hillary Clinton, the US secretary of state, spoke to reiterate that this would change America's policy towards the war. Mr Assad is under severe pressure as the fighting comes closer to Damascus. The secretary-general of the Arab League, Nabil al-Arabi, said in an interview with AFP news agency that he thought the Assad regime could collapse "at any time." "Facts on the ground indicate very clearly now that the Syrian opposition is gaining, politically and militarily," he said. Every day they are gaining something. The United Nations also announced it was sending all non-essential, non-Syrian staff out of the country, and halting trips outside of the capital, because of the worsening security situation there. The reported defection of Mr Makdessi will add to the pressure on Mr Assad. Mr Makdessi was previously the press secretary of the Syrian embassy in London. Syria's air force has been bombing districts in southern Damascus which have fallen into rebel hands, and it has been unable to fully secure the airport and airport road. Although the airport reopened at the weekend, an Egyptair plane from Cairo was turned back on Monday afternoon after being informed security was not assured. However, the regime itself repeated denials yesterday that it would use its chemical weapons arsenal. "Syria confirms repeatedly it will never, under any circumstances, use chemical weapons against its own people, if such weapons exist," an official said.