The US on Monday suspended negotiations with Russia on reviving the stalled ceasefire in Syria and on cooperating militarily to target jihadis in the country.
"This is not a decision that was taken lightly," US State Department spokesman John Kirby said.
Kirby attributed the decision to shortcomings on Russia's side. "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."
The US statement also cites Russia and Syria's intensified military attacks against civilian areas, infrastructure and hospitals, including an attack on an aid convoy last month.
Washington is calling home personnel sent to Geneva to set-up a "Joint Implementation Center" with Russian officers to plan coordinated strikes. US diplomats will also suspend discussions with Russia on reviving the September 9 deal reached between US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov.
Under that protocol, a truce 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.
The US decision "evokes disappointment," a high-ranking source in the Russian Foreign Ministry told the Interfax news agency. The undisclosed source said the ministry would need to further analyze the US decision before officially responding.
Moscow has said in recent days that the US had failed to separate "moderate" anti-Assad rebels from Al Qaeda-linked jihadis.
Meanwhile, Russian President Vladimir Putin has suspended a treaty with Washington on cleaning up weapons-grade plutonium, citing issues including the sanctions placed on Moscow since the annexation of Crimea.
jbh/msh (AFP, dpa)