The most senior Syrian politician to defect to the opposition has told the BBC the regime will not hesitate to use chemical weapons if it is cornered.
Nawaf Fares, ex-ambassador to Iraq, said unconfirmed reports indicated such weapons might have already been used.
He also said that major bombings across Syria had been orchestrated by the regime in collaboration with al-Qaeda.
Meanwhile, UN-Arab League envoy Kofi Annan is due to hold talks with Russian President Vladimir Putin on Syria.
Russia is a key ally of Syria and the meeting comes amid mounting pressure for tougher international action against the country.
Syria has been in turmoil since March last year when an uprising against the government of President Bashar al-Assad began.
UN Secretary General Ban Ki-moon is due in Beijing for talks with the Chinese leadership, which has joined Russia in vetoing sanctions.
Diplomat efforts come as UN officials complained of huge obstacles put in the way of its aid operation in Syria.
Syria is known to have a significant stockpile of chemical weapons. There have been growing concerns in neighbouring countries and among Western governments about the security of such weapons should the regime fall.
Asked if he thought President Assad might use chemical weapons against the opposition, Mr Fares told BBC security correspondent Frank Gardner in an interview in Qatar that he would not rule it out, describing Mr Assad as "a wounded wolf and cornered".
"There is information, unconfirmed information of course, that chemical weapons have been used partially in the city of Homs," he said.
Mr Fares's claim that Sunni Muslim militants in al-Qaeda are collaborating with a regime dominated by those from the minority Allawite sect will surprise many.
Challenged on his view that al-Qaeda was collaborating with the regime despite this, Mr Fares said: "There is enough evidence in history that lots of enemies meet when their interests meet."
He added: "Al-Qaeda is searching for space to move and means of support, the regime is looking for ways to terrorise the Syrian people."
Mr Fares is the most prominent politician to defect since the uprising against President Assad began.
He has held senior positions in the ruling Baath Party and powerful security services, and served as governor in several provinces.
Source: BBC News