A former Syrian forensic medicine expert has provided evidence that the Assad regime used chemical weapons, opposition forces say.
Abdeltawwab Shahrour, head of the forensic medicine committee in Aleppo, claims there was a chemical attack in Khan al-Assal, Aleppo, on March 19, said Istanbul-based opposition coalition spokeswoman Sarah Karkour.
Mr Shahrour, who has defected to Turkey, has documents proving the attack took place and eyewitness accounts from police authorities that contradicte the administration's version of events, a second opposition official said.
Both the government and rebels have blamed each other for what they say was an attack involving chemical weapons. More than two dozen people died.
Russia, which alongside Iran is Syria's closest ally and chief arms supplier, said in July its own scientific analysis indicated the attack had involved the nerve agent sarin and had most likely been carried out by the rebels.
Both sides deny using chemical weapons.
Dr Shahrour was expected to reveal the details of the attack during a press conference in Turkey but Syrian National Council spokesperson Khaled Saleh said he was unable to appear due to "security concerns".
Mr Saleh added that the national council has received information in the last few days that three government convoys were carrying chemical weapons and one had reached a military airport.
He said: "We have serious concerns based on the information that we have received from sources inside the Assad army who are sympathetic to the revolution that Assad might be considering using those chemical weapons against innocent civilians."
A team of UN experts who visited Syria last month to investigate allegations of chemicals weapons had originally planned to visit Khan al-Assal.
However, it ended up focusing on a much larger apparent poison gas attack which killed hundreds of civilians in suburbs of the capital Damascus on August 21.
The new opposition claims come as France bolstered calls for military action in Syria by saying it had evidence the regime was behind the "massive and co-ordinated" chemical attack in Damascus.
Syria's government called the allegations "illogical" and warned it would retaliate if France took military action.
The US government has also said evidence points to deadly sarin gas being used to kill 1,429 people in the August attack.
US President Barack Obama has been meeting political figures such as Senator John McCain as he tries to galvanise support for military strikes ahead of a debate in Congress.