By Dasha Afanasieva
ISTANBUL, Sept 3 (Reuters) - A forensic expert with evidence President Bashar al-Assad's government used chemical weapons near Aleppo in March has defected to Turkey, the Syrian opposition said on Tuesday, but the medic failed to appear at a planned news conference.
Washington is considering military action over an alleged chemical weapons attack near Damascus last month that killed hundreds. Evidence of their past use by government forces could prove important.
The Istanbul-based Syrian opposition coalition said 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.
But Shahrour, who opposition officials said had been under close protection since defecting two weeks ago, failed to appear at a news conference planned by the opposition on Tuesday. Coalition spokesman Khaled Saleh said security concerns had kept him away and that he would instead appear in the coming days.
Turkish officials were not immediately available to confirm the defection.
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 in March 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. (Additional reporting by Yesim Dikmen; Writing by Nick Tattersall; Editing by Ralph Boulton)