The Trump administration issued a blistering statement on Sunday accusing Russia of killing innocent civilians in Syria as forces loyal to Syrian President Bashar Assad advance on a key rebel enclave.

"After repeatedly delaying the passage of United Nations Security Council Resolution (UNSCR) 2401, which demanded a 30-day cessation of hostilities across Syria, Russia has gone on to ignore its terms and to kill innocent civilians under the false auspices of counterterrorism operations," White House press secretary Sarah Huckabee Sanders said in the statement.

The statement comes after Moscow blocked a United Nations (U.N.) resolution in February that would have seen a 30-day ceasefire imposed on parts of Eastern Ghouta, a rebel-held area east of Damascus.

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Hundreds of people have been killed over the past several weeks as pro-Assad forces, supported by Russia, work to retake the area, which is the last major rebel stronghold in the country. Government forces have retaken 25 percent of the area, according to the BBC, leading to a mass exodus of civilians.

The U.N. said in a statement on Sunday that it was concerned about the violence and that "collective punishment of civilians is simply unacceptable."

Sanders in her statement called on Assad's forces to immediately halt its offensive.

"The civilized world must not tolerate the Assad regime’s continued use of chemical weapons," Sanders said, referring to recent reports that the Assad government has used chemical weapons against the rebel-held areas. Assad has denied the allegations.

The Trump administration had earlier in February called on Moscow to back the U.N.-led ceasefire in order to allow humanitarian aid into the besieged parts of the country.

Russia, which says it's aiding in the fight against terrorist groups in Syria, said such a ceasefire was unrealistic.