A forensic medicine specialist has provided evidence that forces loyal to Syrian President Bashar Assad carried out a chemical weapons attack against civilians and rebel fighters in March, Syrian opposition forces claimed Tuesday.

According to Reuters, Abdeltawwab Shahrour — a Syrian national, currently residing in Turkey, who is the former head of Aleppo’s forensic medicine committee — said he possessed proof that chemical weapons were used by Syrian soldiers during a March 19 assault on Khan al-Assal, just outside Aleppo. Over 30 people were reported to have been killed in that attack.

A spokeswoman for a Syrian opposition group said Shahrour had documents that proved an unconventional strike had taken place. The spokeswoman further stated that many witnesses who were at the site at the time of the alleged attack testified that chemical weapons were used by regime forces.

Syrian authorities have repeatedly denied such accusations, instead claiming that the rebels had been the ones to carry out the attacks.

Earlier this week, Syria’s ambassador to the UN, Bashar Jaafari, said that the Syrian government had requested that the UN investigate the attack and assist in identifying who was behind it.

“They did not raise a finger in the media to say that what happened in Khan al-Assal was wrong,” Jaafari said, referring to Western nations.

The regime, whose allegation was backed by its ally Russia, said 31 people were killed in what it said was a rebel attack, including 21 civilians and 10 soldiers. Rebel forces denied the allegation and blamed Assad’s forces.

The reported attack was in an area just west of the city of Aleppo that had seen fierce fighting for weeks before rebels took over a sprawling government complex there last month.

The facility included several military posts and a police academy that Assad’s forces had turned into a military base that regularly fired shells at nearby villages.

The IDF’s top intelligence analyst, Itai Brun, said in April that the army was quite certain that Assad’s forces deployed chemical weapons against rebel forces in the March 19 attack.

Allegations of chemical weapons use by regime forces have been surfacing over the past week, specifically with regard to an alleged August 21 sarin gas attack that according to US claims left over 1,400 dead.

US President Barack Obama recently backed off imminent military action against Syria over that attack, instead opting to await Congressional approval.