The US has broken off talks with Russia on trying to end the violence in Syria and accuses Moscow of not living up to its commitments to halt the violence and deliver humanitarian aid to besieged communities.
The confirmation the US-Russian talks on Syria have collapsed suggests there is little hope of a diplomatic solution emerging soon to end a 5-1/2-year civil war that has killed hundreds of thousands and displaced 11 million.
"The United States is suspending its participation in bilateral channels with Russia that were established to sustain the cessation of hostilities," State Department spokesman John Kirby said in a statement.
Secretary of State John Kerry last spoke to his Russian counterpart, Sergei Lavrov, on Saturday, a senior State Department official said, after Kerry threatened last week to walk away from the talks.
In Moscow, Foreign Ministry spokeswoman Maria Zakharova told Russian media the US was trying to shift blame onto Moscow, which in recent days had tried to sustain the agreement.
But Syrian President Bashar al-Assad's forces, supported by Iranian-backed militia and Russian air power, have since last week stepped up their offensive against rebel-held areas of Aleppo, bombing hospitals and damaging water supplies.
The end of the talks could trigger deeper US consideration of military options such as providing more sophisticated arms, logistical support and training to Syrian rebel groups, either directly or via Gulf Arab states or Turkey.
But the speed with which the ceasefire collapsed - after a UN aid convoy was bombed in Syria - appeared to take some officials in the Obama administration by surprise and without a clear plan on the immediate way forward.
Just before Monday's announcement, Russian President Vladimir Putin suspended a treaty with Washington on cleaning up weapons-grade plutonium, signalling he was willing to use nuclear disarmament as a new bargaining chip in disputes with the United States over Ukraine and Syria.
The senior State Department official rejected links between the suspension of talks on Syria and Putin's announcement.
Kirby said the US would continue to communicate with the Russian military to ensure that American and Russian aircraft operating in Syria do not come into conflict with each other.
But he said the US would withdraw all personnel it had dispatched to prepare for military co-operation with Russia under the latest ceasefire agreement.
Meanwhile, UN diplomats said the United Nations Security Council would begin to negotiate on Monday a draft resolution that urges Russia and the US to ensure an immediate truce in Aleppo.
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