The U.S. is calling on Russia to end its support for Syrian President Bashar Assad after dozens of people were killed in an alleged chemical attack on Saturday.

State Department spokesperson Heather Nauert said in a statement that U.S. officials are closely following the “disturbing reports … regarding another alleged chemical weapons attack, this time targeting a hospital in Douma, Syria.”

“These reports, if confirmed, are horrifying and demand an immediate response by the international community,” Nauert said.

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“The United States continues to use all efforts available to hold those who use chemical weapons, in Syria and otherwise, accountable,” she added. “The Assad regime and its backers must be held accountable and any further attacks prevented immediately.”

Nauert also said that Russia “ultimately bears responsibility for these brutal attacks.”

“The United States calls on Russia to end this unmitigated support immediately and work with the international community to prevent further, barbaric chemical weapons attacks,” she added.

Damascus has denied responsibility for the attack, Reuters reported.

Trump administration officials told The Associated Press in February that they believed forces loyal to Assad might have been in the process of working on new, or more refined chemical weapons.

Officials also said at the time that it was “highly likely” that Syria kept a stockpile of weapons after a 2013 deal to destroy its chemical program, according to the AP.

President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE said last year that reports of a deadly gas attack by forces loyal to Assad had “crossed a lot of lines.”

Trump indicated during a joint press conference with Jordan’s King Abdullah II last April that the Syrian leader had gone too far with an attack in the rebel-held province of Idlib.

“It crossed a lot of lines for me,” Trump said. “When you kill innocent children, innocent babies, little babies with a chemical gas that is so lethal that people were shocked to hear what gas it was, that crosses many lines beyond the red line. Many, many lines.”

“I will tell you, what happened yesterday is unacceptable to me,” Trump added.

Trump also said reports of women and children who had died had a "big impact" on him and caused him to rethink his strategy toward Assad.

“I do change. I am flexible. I am proud of that flexibility,” Trump said. “I will tell you that attack on children yesterday had a big impact on me. Big impact. It was a horrible, horrible thing. I've been watching it and seeing it, and it does not get any worse than that. I have that flexibility. And it is very, very possible, and I will tell you it is already happened, that my attitude toward Syria and Assad has changed very much.”

— This report was updated at 7:02 a.m.