The United States said on Monday that it has suspended talks with Russia on efforts to revive a failed ceasefire in war-ravaged Syria.
"The United States is suspending its participation in bilateral channels with Russia that were established to sustain the cessation of hostilities," a senior US State Department official said.
"This is not a decision that was taken lightly."
— Department of State (@StateDept) October 3, 2016
US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov agreed over a ceasefire that came into effect on September 12 but it collapsed within a week amid bitter recriminations and a surge of fighting in the five-year-old civil war.
Washington has repeatedly accused Moscow of failing to influence Bashar al-Assad to stop strikes on civilian targets.
Moscow, meanwhile, says the United States failed to separate "moderate" anti-Assad rebels from Al Qaeda-linked militants.
Commenting on the US move, Russia's foreign ministry said it regretted the US decision.
It said Washington was trying to shift responsibility for the failure onto Moscow.
"We regret this decision by Washington," foreign ministry spokeswoman Maria Zakharova was quoted as saying by Russian news agencies.
The United States "after failing to fulfil the agreements that they themselves worked out, are trying to shift responsibility on to someone else", she said.
Heavy civilian casualties have been reported since the collapse of the ceasefire as a ground operation was launched by Syrian regime forces against the besieged eastern half of Aleppo, where opposition fighters have been holding out.