The Bashar Assad regime on Thursday reportedly attacked a residential suburb of Damascus with chlorine gas, according to the White Helmets, a pro-opposition Syrian civil defense agency, in the third strike in less than a month. Numerous civilians, including children, were injured in the attack, which targeted the opposition-held city of Duma in the Eastern Ghouta district, White Helmets officials said.

On Jan. 22, at least 21 people were treated for respiratory problems after rockets were fired on the rebel bastion of Eastern Ghouta outside Damascus, which residents and medical sources said contained chlorine.

In a recently released annual report, the White Helmets said that as many as 1,337 civilians had been killed in Eastern Ghouta last year amid ongoing regime attacks.Home to some 400,000 residents, Eastern Ghouta has been under siege for five years, and humanitarian access to the city has been completely cut off. Hundreds are in urgent need of medical attention.

In the past eight months, the Syrian regime has intensified its siege on Eastern Ghouta, making it almost impossible to bring food or medicine into the district and leaving hundreds of patients in need of treatment. In an attempt to bring an end to the fighting, backers of opposing sides last year agreed to four de-escalation" zones in the country. Notably, the district falls within a network of de-escalation zones endorsed by Turkey, Russia and Iran, in which acts of aggression are expressly forbidden. Idlib makes up part of one zone. The other three are in the south, the central province of Homs and the area of Eastern Ghouta, an opposition enclave near Damascus.

The Assad regime has been repeatedly accused of using chemical weapons on civilians. The United Nations found that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015. And last year, the U.N. and others blamed regime forces for an April 2017 sarin gas attack on the rebel-held town of Khan Sheikhun that left scores dead.

The U.S. administration on Thursday accused the regime of producing and using new kinds of weapons to deliver deadly chemicals despite committing to abolish its program in 2013, and said the world must find a way to stop it. Raising the alarm about the continued threat, U.S. officials said it was highly likely that Assad kept a hidden stockpile of chemical weapons after 2013 that he failed to properly disclose. They said information gathered from recent alleged attacks also suggested that Assad retained a "continued production capacity," also banned under the 2013 deal.

Years of efforts from two U.S. presidents have failed to end the harrowing reports of chemical weapons in Syria. Under President Barack Obama, the United States stopped short of striking Assad forces in response, but brokered a deal with Russia to rid Syria of its stockpiles. After another alleged attack in April 2017, President Donald Trump ordered a retaliatory missile strike, but 10 months later, the U.S. and international observers say the weapons are still used.