Syrian rebels claim that the army of President Bashar Assad again used poison gas in an attack on rebel forces in the Damascus area Thursday, an Israel Radio report said.

The report was based on rebel sources, who said the attack took place in the Jobar neighborhood of the capital. The rebel sources said they could not tell what kind of chemical weaponry had been used, but that it was a poison gas of some kind, and was causing injuries, including breathing difficulties.

Footage posted to YouTube, and broadcast on Al Arabiya TV, showed a young man having trouble breathing, and receiving medical treatment. There were no immediate reports of fatalities.

The news broke amid word that a United Nations investigative team has amassed a “wealth” of evidence indicating that Syrian President Bashar Assad was responsible for the chemical attack that took place on August 21, killing as many 1,400 people.

Foreign Policy magazine reported that the investigators will present their findings on Monday to UN Secretary General Ban Ki-moon, and will offer strong circumstantial evidence that government forces were responsible for the deadly attack. The report, quoting an unnamed senior Western official, is based on the team’s examination of spent rocket casings, ammunition, and soil, blood, and urine samples. It will stop short of directly accusing Assad of perpetrating the attack on his own people.

The investigators have “gotten very rich samples — biomedical and environmental — and they have interviewed victims, doctors and nurses,” the official was quoted as saying, adding that “they are very happy with the wealth of evidence they got.”

US officials expected the report to confirm US allegations that sarin gas was used in the attack.

The UN delegation consisted of 20 members and was led by Swedish chemical weapons expert Ake Sellstrom. They arrived in Syria on August 18 to investigate allegations that Syrian President Bashar Assad had employed chemical weapons in earlier attacks.

In June, US President Barack Obama said he had “conclusive evidence” that Assad was responsible for the chemical weapons used in those earlier attacks.

Three days after the delegation arrived in Damascus, the large chemical attack was launched, allegedly by government forces. Several days later, on August 26, the investigative team came under sniper fire when it arrived at the scene of the attack to conduct an examination.

One of the UN vehicles was damaged, but nobody was injured by the gunfire.

The team left Syria on August 31.

Assad has all along denied responsibility for the alleged chemical weapons attacks, claiming that opposition forces were behind them all.

Russian President Vladimir Putin, Assad’s key ally, wrote in an op-ed piece published Wednesday that “no one doubts that poison gas was used in Syria. But there is every reason to believe it was used not by the Syrian army, but by opposition forces, to provoke intervention by their powerful foreign patrons.”

Earlier this week, US Secretary of State John Kerry, in an apparent offhand remark, suggested that direct US military intervention could be averted if Syria agreed to place all of its chemical weapons under international control. Russia, which has long been an ally of the Assad regime, pitched the idea to Syria, whose foreign minister, Walid Moallem, welcomed the proposal.

The initiative prompted Obama to announce that he would be putting on the back burner plans to appeal to US lawmakers for approval for a military strike.