It happened exactly one year after President Donald Trump ordered a cruise missile strike on Syrian forces.
Dozens were brutally killed and hundreds more were injured from a suspected chemical attack in Syria on Saturday.
Estimates of the death toll range from about 40 to more than 150. At least 70 people "suffocated to death" and hundreds more were injured in Douma, a suburb of Damascus in the Eastern Ghouta area, the BBC and Al Jazeera reported, citing the White Helmets and other local rescue groups.
A pro-opposition group called the Ghouta Media Center said the attack was carried out by a helicopter, which dropped a barrel bomb containing sarin gas, the BBC reported.
"Reports from a number of contacts and medical personnel on the ground indicate a potentially high number of casualties, including among families hiding in shelters," The US State Department said in a statement late Saturday night.
Several very graphic images and videos have been posted on social media showing dozens of dead civilians with white foam coming out of their noses and mouths.
A few videos show rooms full of dead women and children who had been hiding in basements from Russian and Syrian regime airstrikes. One disturbing video shows rescue workers aiding a young girl who is choking and screaming.
The "fatalities occurred not just underground, but above ground - suggesting potent chlorine (which sinks) is now a less likely culprit," Charles Lister, a senior fellow at the Middle East Institute, tweeted.
The US State Department said Syrian President Bashar al-Assad's regime's "history of using chemical weapons against its own people is not in dispute," and referenced a sarin gas attack that Assad's forces conducted on April 4, 2017 in Khan Sheikhoun, which killed at least 70 people.
The Syrian regime has denied the accusations, blaming opposition rebels for that attack.
The attack in Douma on Saturday came one year after President Donald Trump responded to the sarin gas attack on Khan Sheikhoun by firing 59 Tomahawk cruise missiles at Syrian regime forces.
"The Assad regime and its backers must be held accountable and any further attacks prevented immediately," the State Department said. "Russia, with its unwavering support for the regime, ultimately bears responsibility for these brutal attacks, targeting of countless civilians, and the suffocation of Syria’s most vulnerable communities with chemical weapons. By shielding its ally Syria, Russia has breached its commitments to the United Nations as a framework guarantor. It has betrayed the Chemical Weapons Convention and UN Security Council Resolution 2118."
Russia "refuted the information," dismissing the news, Reuters reported.
"Make no mistake, in addition to being yet another act of sheer brutality, this was undoubtedly intended - in part - as a test of Western resolve in
#Syria," Lister tweeted.
Syrian and Russian airstrikes in Eastern Ghouta have killed more than 1,600 civilians since February 18, according to AFP.
Trump tweeted about the attack Sunday morning:
"Many dead, including women and children, in mindless CHEMICAL attack in Syria. 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! ... If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!"