At least 40 people are reported dead in Syria -- including women and children -- after a suspected chemical gas attack on a rebel-held town near the capital on Saturday, as the State Department said the reports, if confirmed, would demand "an immediate response."

The Syrian American Medical Society and opposition-linked Syrian Civil Defense said in a joint statement that at least 40 people had died in the attack in Douma, about 10 miles east of Damascus, and over 500 people, mostly women and children, were injured and brought to medical centers.

The injuries included difficulty breathing, foaming at the mouth, and burning of the eyes, according to the organization. It said patients gave off a chlorine-like smell and some had blue skin, a sign of oxygen deprivation.

The alleged attack occurred amid a resumed offensive by Syrian government forces after the collapse of a truce. The Russian-backed Syrian government denied that its forces had launched any chemical attack, Reuters reported. The government said rebels in Douma were in a state of collapse and spreading false news.

None of the reports could be immediately independently confirmed, both the Associated Press and Reuters said.

President Trump said on Twitter that the Syrian Army needs to open the area "immediately" for medical help and verification.

"Many dead, including women and children, in mindless CHEMICAL attack in Syria," Trump tweeted. "Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!"

First responders said they found families suffocated in their homes and shelters, with foam on their mouths. The opposition-linked Syrian Civil Defense were able to document 42 fatalities but were impeded from searching further by strong odors that gave their rescuers difficulties breathing, said Siraj Mahmoud, a spokesman for the group, which is known as the White Helmets.

"Until this minute, no one has been able to find out the kind of agent that was used," Mahmoud said in a video statement.

Videos posted online by the White Helmets showed victims, including toddlers in diapers, breathing through oxygen masks at makeshift hospitals.

The Syrian government, in a statement posted on the state-run news agency SANA, strongly denied the allegations. It said the claims were "fabrications" by the Army of Islam, calling it a "failed attempt" to impede government advances.

"The army, which is advancing rapidly and with determination, does not need to use any kind of chemical agents," the statement said.

LARGEST SYRIAN REBEL GROUP STARTS LEAVING DAMASCUS ENCLAVE

Syrian government forces resumed their offensive on rebel-held Douma on Friday afternoon after a 10-day truce collapsed over disagreement regarding the evacuation of Army of Islam fighters.

Violence resumed days after hundreds of opposition fighters and their relatives left Douma toward rebel-held areas in northern Syria. Douma is the last rebel stronghold in eastern Ghouta.

Syrian state media said that rebels have agreed to give up their last foothold in the eastern Ghouta region outside Damascus and withdraw to north Syria.

The SANA news agency says the Army of Islam group agreed Sunday to leave Douma, three days after the government resumed its assault on the besieged town. Buses have also been sent to the town to pick up prisoners freed by the rebel group and to transport rebel fighters to opposition-held territory in north Syria, the state media outlet reported.

The alleged gas attack in Douma comes almost exactly a year after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people. That attack prompted the U.S. to launch several dozen Tomahawk cruise missiles at a Syrian air base.

Trump said the attack in 2017 was meant to deter further Syrian use of illegal weapons, , and recently said he would “ideally” start to bring home U.S. troops from Syria within a six-month period.

TRUMP 'IDEALLY' WANTS US TROOPS OUT OF SYRIA WITHIN 6 MONTHS

U.S. State Department spokeswoman Heather Nauert said that Washington was closely following "disturbing reports" of the alleged chemical weapons attack in Douma.

"These reports, if confirmed, are horrifying and demand an immediate response by the international community," she said in a statement late Saturday.

An emergency meeting of United Nations Security Council is expected Monday in response to reported Syria chemical attack, the UK Mission to the United Nations said on Twitter.

The Syrian government and its ally, Russia, denied any involvement in the alleged gas attack.

Douma is in the suburbs of Damascus known as eastern Ghouta. A chemical attack in eastern Ghouta in 2013 that was widely blamed on government forces killed hundreds of people, prompting the U.S. to threaten military action before later backing down.

Syria denies ever using chemical weapons during the seven-year civil war, and says it eliminated its chemical arsenal under a 2013 agreement brokered by the U.S. and Russia after the attack in eastern Ghouta.

The Associated Press contributed to this report.