ISTANBUL — A Syrian forensic medicine expert with evidence that President Bashar Assad's administration used chemical weapons in an attack near Aleppo in March has defected to Turkey, the opposition said on Tuesday.

Abdeltawwab Shahrour, head of the forensic medicine committee in Aleppo, would make public his evidence of the March 19 chemical attack in Khan al-Assal, Istanbul-based opposition coalition spokeswoman Sarah Karkour said.

Shahrour had documents proving that a chemical weapons attack took place and eye-witness accounts from police authorities that contradicted the administration's version of events, a second opposition official said.

The attack at Khan al-Assal in the northern province of Aleppo killed more than two dozen people. Both the government and rebels have blamed each other for what they say was an attack involving chemical weapons.

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.

A team of U.N. experts who visited Syria last month to investigate allegations of chemicals weapons had originally planned to visit Khan al-Assal but ended up focusing on a much larger apparent poison gas attack which killed hundreds of civilians in suburbs of the capital Damascus on Aug. 21.