The United States blasted Russia on Thursday over the bombing of Syria's Eastern Ghouta, saying it has a "unique responsibility" for the more than 400 deaths in the Damascus suburb.
After fresh air and rocket strikes by Syrian government forces killed another 46, State Department Spokeswoman Heather Nauert blamed Russia for not reeling in the regime of its ally President Bashar al-Assad.
"Without Russia backing Syria, the devastation and the deaths would certainly not be occurring," Nauert told journalists.
"It is a good reminder that Russia bears a unique responsibility for what is taking place there."
"What are they doing to stop the devastation, the deaths, the murders that are taking place in Syria?"
The criticism came hours after the UN Security Council failed to agree on a new draft resolution for a 30-day ceasefire in Eastern Ghouta to allow aid to get in to the civilian population stuck there.
Nauert blamed Moscow for again "throwing a wrench" into the negotiations to block the ceasefire.
Nauert said the assault on Eastern Ghouta, which began on Sunday, showed the failure of the Astana process, in which Russia, Iran, and Turkey sought to delineate de-escalation zones in Syria, including one for Eastern Ghouta.
"It shows what a farce this deescalation zone has become," Nauert said.
She said that US Secretary of State Rex Tillerson is "highly focused" on the Eastern Ghouta situation in regular talks with counterparts.
But she was unable to detail what options the United States has to halt the attacks, urging media to be more aggressive in publicizing the humanitarian situation.