The US says it has stopped discussions with Russia about Syria, blaming the country for repeatedly targeting civilians there.
The two had been working on a plan to end the civil war.
However, the White House has said it has run out of patience with Russia after relations deteriorated yesterday when the US State Department announced that bilateral discussions were being stopped.
The move had been threatened last week by Secretary of State John Kerry, furious at repeated airstrikes on rebel-held areas of Aleppo.
Hundreds of civilians are thought to have been killed in bombardments of the city in recent weeks.
America is accusing Russia of "failing to live up to its own commitments" under international law, namely sticking to the terms of an agreement made in September to restore a faltering ceasefire.
The US expressed a desire to the make sure humanitarian aid could reach cities under siege, saying that halting talks was "not a decision we took lightly".
A statement accused Russia of being "unwilling or unable to ensure Syria regime adherence to arrangements to which Moscow agreed".
"Rather," the US said, "Russia and the Syrian regime have chosen to pursue a military course inconsistent with the cessation of hostilities, as demonstrated by intensified attacks against civilian areas, targeting of critical infrastructure such as hospitals, preventing humanitarian aid from reaching civilians in need."
In response, Russia has said it regrets America's decision, insisting it has been attempting to sustain the agreement and throwing the accusation back at the White House.
"After failing to fulfill the agreements that they themselves worked out, they are trying to shift responsibility on to someone else," said Russia's foreign ministry.
The US also says it is withdrawing personnel it had sent to take part in the creation of a joint US-Russia centre.
Military strategy, as well as intelligence, were to be run from the centre.
In a statement, Moscow added: "It all essentially came down to a simple question: who are Jabhat al Nusra, who is behind them and why can't Washington fulfill its promise to divide the terrorists from the so-called moderate opposition?"
The White House said the suspension would not affect communications between the two countries on counter-terrorism operations in Syria.
In a day of high tensions, President Vladimir Putin earlier suspended a deal with the US on the disposal of weapons-grade plutonium.
The White House has said it regrets Russia's decision to put that agreement on hold.