The US has said it is suspending talks with Russia over Syria, accusing Moscow of having "failed to live up" to its commitments under a ceasefire deal.
Washington blamed Russia and the Syrian government for intensifying their attacks against civilians.
Last week, the US warned it would halt the talks unless Moscow stops bombing the city of Aleppo.
Russia said it regretted the US move, accusing it of shifting the blame for the collapse of last month's truce.
Aleppo, Syria's largest city in the north, has come under heavy aerial bombardment since the end of the ceasefire two weeks ago.
The main trauma hospital in the rebel-held eastern part of the city was hit in an air strike for the third time in a week, activists said on Monday.
Hundreds of people, including children, have died since government forces launched an offensive to take full control of Aleppo after the week-long truce lapsed.
Some 250,000 people are trapped in eastern Aleppo.
In a statement, state department spokesman John Kirby said: "The United States is suspending its participation in bilateral channels with Russia that were established to sustain the cessation of hostilities.
"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.
"Rather, Russia and the Syrian regime have chosen to pursue a military course," Mr Kirby said, admitting that "this is not a decision that was taken lightly".
He said Moscow and Syrian President Bashar al-Assad's troops had been "targeting of critical infrastructure such as hospitals, and preventing humanitarian aid from reaching civilians in need, including through the 19 September attack on a humanitarian aid convoy".
Moscow strongly denies involvement of its own or Syrian planes in the deadly aid convoy strike, and says the incident was caused by fire on the ground and not by an air strike.
In response to the US suspension of the talks, Russian foreign ministry spokeswoman Maria Zakharova said: "We regret this decision by Washington."
"Washington simply did not fulfil the key condition of the agreement to improve the humanitarian condition around Aleppo.
"After failing to fulfil the agreements that they themselves worked out, they are trying to shift responsibility on to someone else," Ms Zakharova said.
She also said that the US had failed to divide jihadist groups in Syria from the moderate opposition.
Russia and the US were due to convene in Geneva to try to co-ordinate air strikes against jihadist groups, but American officials were told to return home.
The US also said that it would withdraw personnel "that had been dispatched in anticipation of the possible establishment of the Joint (US-Russian) Implementation Centre".
However, the two sides would keep talking about counter-terrorism operations in Syria to avoid unnecessary clashes.