EASTERN GHOUTA, Syria/ANKARA 

The Assad regime on Thursday attacked a residential suburb of Damascus with chlorine gas, according to the White Helmets, a pro-opposition Syrian civil defense agency.

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.

Last month, the Assad regime targeted residential parts of Duma with chlorine gas, leading to numerous civilian casualties.

The U.S. State Department voiced extreme concern over the reported attack, noting that if it is confirmed it would be the third chemical attack in the past 30 days in east Ghouta. 

"We take the allegations of chemical weapons use very seriously and are working with our partners on the ground to investigate the reports," State Department spokeswoman Heather Nauert told reporters.

Nauert urged Moscow to reign in its ally in Damascus, whom she said Russia has influence over. 

"I think it's clear that Russia does have influence, but Russia is making the wrong choice," she said. "To allow the Syrian regime to use chemical or other weapons against its people yet again is absolutely unconscionable." 

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 attacks by regime forces.

Home to roughly 400,000 residents, Eastern Ghouta, a suburb of Damascus, has remained under a crippling regime siege for the last five years.

Over the past eight months, the Assad regime has stepped up its siege on Eastern Ghouta, preventing the delivery of food or medicine into the district and leaving hundreds of medical patients in need of treatment.

Notably, Eastern Ghouta falls within a network of de-escalation zones -- endorsed by Turkey, Russia and Iran -- in which acts of aggression are expressly forbidden.