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, which has been in turmoil since March last year when an attempted uprising against the government of President Bashar al-Assad began.



The UN observer mission's mandate in Syria comes on to an end on Friday and a new UN resolution is needed to renew it.



Mr Fares said President Assad would not relinquish power peacefully. He said he would only be ousted by force "even if he will have to eradicate the entire Syrian people".




Mr Fares told the BBC's Frank Gardner that Mr Assad would be removed "only by force"

Mr Fares's claim that Sunni Muslim militants in al-Qaeda are collaborating with a regime dominated by those from the Shia 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.



He told BBC security correspondent Frank Gardner that the government "will inevitably fall".



"It is absolutely sure that this government will fall in a short time. We wish for this time to be short so that more sacrifices will not be paid by the Syrian people," he said.



'Wounded wolf'



Syria is known to have a significant stockpile of chemical weapons. There have been growing concerns in neighbouring countries and in key Western governments about the security of such weapons should the regime fall.



Asked if he thought President Assad might us chemical weapons against the opposition, Mr Fares said 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.



"I have absolute conviction that if the circle... becomes tighter on the regime, the regime will not hesitate to use chemical weapons."




Kofi Annan, right, held talks with Russian Foreign Minister Sergei Lavrov on Monday

Violence is continuing to spread across the country and in the capital Damascus as rebels - now better equipped and more organised - confront the army and government-backed militia.



Witnesses say the capital is currently seeing the biggest military deployment in the 16-month uprising.



Activists say that troops backed by armoured vehicles have entered the Midan district to try to dislodge rebels.



Mr Fares said that the family of President Assad were "clinging to power".



"I know the mentality of Bashar al-Assad , I now the mentality of [his late father] Hafez al-Assad, and the entire Syrian people understand that these people... believe they will live forever as rulers of Syria," he said.



"It doesn't occur to any Syrian, not only me, that Bashar al-Assad will let go of power through political interventions - this is impossible. He will be ousted only by force... he will not relinquish power peacefully."



Mr Fares was dismissive of UN-Arab League envoy Kofi Annan's current visit to Russia, saying success there was "impossible".



"The Kofi Annan plan has failed from the beginning," he said.



"Several months have passed so far and... the regime hasn't implemented a single article of this plan."



US Secretary of State Hillary Clinton has said there must be a new UN resolution "with consequences" for Damascus if it fails to comply with Mr Annan's peace plan.



Russian Foreign Minister Sergei Lavrov said on Monday that the West had threatened to end the observer mission if Moscow opposed its draft resolution, which threatens more sanctions. He said the attempts contained "elements of blackmail".



Moscow has circulated its own draft resolution calling for the extension of the mission's mandate but without a threat of sanctions.



Russia and China have blocked two previous UN resolutions that condemned President Assad's government for the continuing violence.