The UN Syria envoy has voiced deep disappointment at the collapse of US-Russian talks to revive a Syria ceasefire, but vowed to keep working for a political solution.
"The UN will continue to push energetically for a political solution of the Syrian conflict regardless of the very disappointing outcome of intense and long discussions among two crucial international stakeholders," the office of Staffan de Mistura said in a statement.
The statement came after the United States earlier suspended negotiations with Russia on efforts to revive a failed ceasefire in Syria and set up a joint military cell to target jihadists.
State Department spokesman John Kirby accused Russia and its Syrian ally of stepping up attacks on civilian areas.
"There is nothing more for the US and Russia to talk about with regard to trying to reach an agreement that would reduce the levels of violence inside of Syria. And that's tragic," he said.
Russia's foreign ministry, meanwhile, said it regretted the decision, blaming Washington for trying to shift responsibility for the failure onto Moscow.
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Russia and the US have traded blame for the collapse last month of a short-lived ceasefire deal that would have marked the first step in a new effort to end the war that has killed 300,000 people since 2011.
Mr De Mistura, who had hoped the ceasefire would help revive long-stalled Syria peace talks, "deeply regrets the fact that the American-Russian bilateral talks on reviving the cessation of hostilities in Aleppo, in particular, and Syria in general, did not reach a positive conclusion," his office said.
Mr Kirby said the Russian and US militaries would continue to use a communications channel set up to ensure their forces do not get in each other's way during "counter-terrorism operations in Syria."
But the US was calling back home personnel sent to Geneva to set up a "Joint Implementation Centre" with Russian officers to plan coordinated strikes.
US diplomats will suspend discussions with Russia on reviving the 9 September ceasefire deal.
Mr De Mistura's office stressed though that the UN-backed humanitarian taskforce for Syria "will continue its hard work in order to assist the unimpeded delivery of aid to vulnerable civilians in Syria".
"The UN will never abandon the Syrian people to a destiny of endless violent conflict," it stressed.