The US has ended negotiations with Russia concerning the ceasefire agreement with Syria, it said today.
A peace plan that was announced last month deteriorated after Russia decided on Monday that it will ditch an agreement with the US to dispose of weapons-grade plutonium.
The US State Department said Russia was "unwilling or unable" to convince the Syrian leadership to agree the ceasefire, and it had instead turned on the opposition in Syria.
On Sunday, the Syrian regime moved in on parts of Aleppo held by rebels, after carrying out a bombing campaign on the city.
Speaking anonymously, a US intelligence official told Reuters that the Aleppo air strikes used "deadly munitions", including barrel bombs, thermobaric bombs, incendiary munitions, cluster bombs and bunker busters.
"The United States is suspending its participation in bilateral channels with Russia that were established to sustain the cessation of hostilities," US State Department spokesman John Kirby said in a statement.
"Unfortunately, Russia failed to live up to its own commitments ...and was also either unwilling or unable to ensure Syrian regime adherence to the arrangements to which Moscow agreed."
As part of the original ceasefire, the US had offered to work with Russia on air strikes against Syrian jihadi groups, if Russia would try to convince the Assad regime to stop attacks in contested parts of Syria.
But the ceasefire collapsed after a US aircraft attacked Syrian forces, followed by Russian and Syrian planes bombing Aleppo, striking an aid convoy.