Nawaf Fares, ex-ambassador to Iraq, said unconfirmed reports indicated such weapons may 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 Syrian and the meeting comes amid mounting pressure for tougher international action on Syria.
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 Bashar al-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 was the most prominent politician to defect since the uprising against President Assad began more than 16 months ago.
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 are reduced," he said.
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