US President Donald Trump on Sunday said there will be a "big price to pay" after what he called a "mindless CHEMICAL attack" in Syria, allegedly involving chlorine gas.
Trump also called Syrian President Bashar al-Assad an "animal."
"President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay," Trump said in a pair of tweets which began with a discussion of the attack in Syria's Eastern Ghouta, where rescue workers alleged that regime loyalists had used chlorine gas.
"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," the president said.
At least 80 civilians have been killed since Friday after the regime launched fresh air raids on rebel-held areas of Eastern Ghouta, according to the Syrian Observatory for Human Rights, a Britain-based monitor.
Syrian state media and the regime's ally Russia denounced claims of chemical use as "fabrications."
"Open area immediately for medical help and verification," Trump said. "Another humanitarian disaster for no reason whatsoever. SICK!"
The latest alleged attack came a year after the rebel-held town of Khan Sheikhun in northwestern Syria was hit by an air strike. A UN-commissioned report said many residents of the town suffered the symptoms of an attack from an illegal nerve agent and more than 80 of them died, convulsed in agony.
Trump responded to that strike three days later, when US warships in the Mediterranean fired 59 cruise missiles at a Syrian airbase.
Assad denied ordering the attack and Russia has continued to give him diplomatic cover at the United Nations.
Trump on Sunday criticized his predecessor Barack Obama for not striking after warning that the use of chemical weapons in Syria was a "red line."
"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!" Trump said.
Separately, a White House security adviser on Sunday refused to rule out US military action after the latest alleged chemical strike.
"This is one of those issues on which every nation, all peoples, have all agreed, and have agreed since World War Two, is an unacceptable practice," White House homeland security adviser Tom Bossert told ABC's "This Week."
Asked whether there will be another US missile strike, Bossert replied: "I wouldn't take anything off the table."
And a prominent Republican senator warned Trump that a failure to act now could leave him in a weakened posture internationally.
"It's a defining moment in his presidency," Lindsey Graham said on ABC. "He has challenged Assad in the past not to use chemical weapons... If it becomes a tweet without meaning, then he has hurt himself in North Korea... (and will) look weak in the eyes of Russia and Iran."
In a joint statement on Wednesday, one year after the Khan Sheikhun attack, the foreign ministers of Britain, France, Germany and the United States said those responsible should be held to account for "these abhorrent attacks in Syria."
The United States has more than 2,000 military personnel in eastern Syria as part of a coalition it leads, providing weapons, training and other support to forces fighting Islamic States jihadists in Syria and neighboring Iraq.
In late March, Trump said those US troops would be coming home "very soon."
On Sunday, Republican Senator Susan Collins told CNN that, in light of new allegations of a chemical strike, "I think the president is going to have to reconsider his plan for an early withdrawal."