Dozens of people have died in a suspected chemical attack in Douma, the last rebel-held town in Syria's eastern Ghouta region, opposition activists and rescuers say.
Opposition-linked first responders, known as the White Helmets, and other activist groups said toxic gas inside barrel bombs dropped by a helicopter over Douma, near Damascus, late on April 7 caused people to suffocate and choke.
There has been no independent verification of the reports, which Damascus and its ally Moscow called a "fabrication."
The United States said Russia, which has given crucial military and diplomatic backing to Syrian President Bashar al-Assad's government throughout Syria’s war, "ultimately bears responsibility" if the attacks are confirmed.
There were conflicting reports on the number of dead in the alleged chemical attack, with the White Helmets reporting between 40 and 70 killed. The group said the victims showed signs of gas poisoning including pupil dilation and foaming at the mouth.
In a joint statement, the relief organization Syrian American Medical Society and Douma's civil defense service said 49 people died in the attack.
The statement said medical centers received more than 500 cases of people suffering breathing difficulties, and that patients gave off a chlorine-like smell.
The U.S. state department said Washington was closely monitoring "very disturbing" reports of the possible "horrifying" new use of chemical weapons by Syrian forces.
Russia, with its "unwavering support" for Syria's government, "ultimately bears responsibility" for the alleged attacks, it said in a statement.
"The [Syrian] regime's history of using chemical weapons against its own people is not in dispute," it added.
A joint inquiry by the UN and the Organization for the Prohibition of Chemical Weapons (OPCW) has found that Syrian government forces had used chlorine as a weapon at least three times during the seven-year conflict.
In April 2017, more than 80 people died in a sarin attack on the rebel-held town of Khan Sheikhoun, and a joint inquiry by the UN and the UN-OPCW mission held the Syrian government responsible. Damascus denies using chemical weapons.
Syria's state news agency SANA said the latest reports of a chemical attack were invented by the Jaish al-Islam rebels who remain in control in Douma, which has been under siege from Russian-backed Syrian government forces.
Russian news agency quoted Major-General Yuri Yevtushenko, head of the Russian peace and reconciliation center in Syria, as saying he "decidedly" rejected the allegations.
"The spread of bogus stories about the use of chlorine and other poisonous substances by [Syrian] government forces continues,” Russia's Foreign Ministry later said in a statement.
The ministry said the aim of "such deceitful speculation" was to "shield terrorists" and to try to “justify possible external uses of force."
The British-based Syrian Observatory for Human Rights said government air strikes had killed 70 civilians in Douma since an intense aerial and ground assault was launched on April 6.
The offensive by Syrian government forces and their allies, which involved weeks of intense bombardment, has left more than 1,600 civilians dead and thousands more wounded in eastern Ghouta since February 18, according to the monitor group.