ISTANBUL (AFP) - The Syrian opposition said on Tuesday it feared a fresh chemical attack by President Bashar al-Assad's regime, after three convoys believed to be filled with chemical weapons were spotted.

"We have had recent information that there were three convoys loaded with chemical weapons (that) left their locations and now we can confirm that two of them have reached their final destination," opposition spokesman Khaled Saleh told a press conference in Istanbul.

Since the weekend, two convoys reached Ezra'a, near to the southern city of Deraa, and the Dommair military airport on the outskirts of Damascus, Mr Saleh said.

"We have very serious concerns, based on information that we received from sources inside Assad's army, that they might be considering using those chemical weapons against innocent civilians," he said.

"We have someone in Damascus who is willing to use chemical weapons at any cost, unfortunately the message that he has been receiving from the international community until now is a green light."

Washington has accused the Assad regime of using chemical weapons in an August 21 attack near Damascus, which US officials say killed more than 1,400 people, including hundreds of children. Mr Assad's government has rejected the claims.

The US and France are pushing for punitive air strikes against the Syrian regime but want to gather international support before taking any military action.

Mr Obama's decision on Saturday to ask Congress to authorise action against Syria lifted the threat last week of any immediate strikes against Mr Assad's government.

The British Parliament last week rejected any intervention in Syria.

Forensic examiner Abdul Tawwab Shahrour, based in the northern city of Aleppo, was due to provide evidence that Syria had used chemical weapons in the city earlier this year but had to postpone his presentation in Istanbul "for security reasons", Mr Saleh said.