The United States has abandoned its effort to convince Russia to impose a ceasefire on the Syrian regime as government forces pursued their relentless onslaught on eastern Aleppo.
Accusing Moscow of abetting Syrian strongman Bashar al-Assad's assault on civilian districts of the city, Washington said it had suspended bilateral talks with Russia on reviving a truce.
"Everybody's patience with Russia has run out," White House spokesman Josh Earnest told reporters.
A State Department official said US Secretary of State John Kerry is "laser-focused" on finding a diplomatic solution to the conflict through multilateral channels.
But his near-daily telephone calls and regular Geneva talks with Russian Foreign Minister Sergei Lavrov on the crisis are over.
"This is not a decision that was taken lightly," State Department spokesman John Kirby said, announcing an end to plans to set up a joint US-Russian military cell to target jihadists.
There was no word on what America's "Plan B" might be despite rumors of tougher US sanctions and talk that Saudi Arabia and Qatar might step up arms shipments to anti-Assad rebels.
At the United Nations, Russia dismissed a French-drafted UN resolution aimed at imposing a ceasefire in Aleppo as having "no chance of working," insisting the priority should be fighting jihadists in Syria.
"I'm not even sure many other council members would like to see a resolution on cessation of hostilities which has no chance of working," Russian Ambassador Vitaly Churkin said.
"If the only effect of that resolution is that the secretary general will start thinking of some monitoring mechanism which is not going to work in the first place, then there is not much sense in having that resolution."
- Back and forth -
The Russian and US militaries will keep a communications channel set up to ensure their forces do not get in each other's way during "counterterrorism operations in Syria," Kirby said.
But US personnel who had been sent to Geneva to set up a "Joint Implementation Center" with Russian officers are coming home.
"We regret this decision by Washington," Russian foreign ministry spokeswoman Maria Zakharova said, Russian news agencies reported.
A truce was declared on September 12 under a deal Kerry and Lavrov signed in Geneva last month, but it collapsed within a week.
Washington accused Moscow of failing to rein in Assad's government forces and of carrying out air strikes on civilian targets, including a UN aid convoy.
Signup to our newsletter and follow us on Facebook and Twitter!
Moscow, meanwhile, says the United States failed to separate "moderate" anti-Assad rebels from jihadists linked to Al-Qaeda.
According to Kirby, Russia was "unwilling or unable to ensure Syrian regime adherence to the arrangements to which Moscow agreed."
The US spokesman accused Moscow and Damascus of targeting hospitals and preventing aid from reaching desperate civilians.
The Russian response was just as stark.
"Washington simply did not fulfil the key condition of the agreement," Zakharova said of Washington's struggle to separate opposition fighters from the jihadist Fateh al-Sham Front.
"After failing to fulfill the agreements that they themselves worked out, they are trying to shift responsibility on to someone else."
Meanwhile, the fighting continued.
Fateh al-Sham Front, which recently changed its name from Al-Nusra Front following a break with Al-Qaeda, confirmed the death of veteran commander Ahmed Salama Mabrouk after the Pentagon said it had targeted him in an air strike.
US officials are "still assessing the results" to determine whether Mabrouk was killed, Pentagon spokesman Peter Cook said, describing him as "one of Al-Qaeda in Syria's most senior leaders and a legacy Al-Qaeda terrorist who previously had ties to Osama bin Laden."
- 'Speechless shock' -
The aid agency Doctors Without Borders reacted with fury to the latest hospital bombings, saying strikes on clinics in Syria and Yemen leave "the global medical community in speechless shock."
That came after an air raid destroyed rebel-held eastern Aleppo's biggest hospital, the M10.
"The hospital is now not usable at all," said Adham Sahloul of the Syrian American Medical Society. "It is not salvageable, per reports from the staff and doctors there."
Three maintenance workers were killed, he said.
Moscow on Monday denied it was bombing hospitals, insisting its air campaign was "highly effective" and had stopped jihadists taking over Syria.
Elsewhere, at least 21 Turkish-backed Syrian rebels fighting the Islamic State group were killed on Sunday by landmines laid by the jihadist group, the Syrian Observatory for Human Rights said.
The toll was the highest loss of life for forces involved in the Turkish-led operation in northern Syria since it began in late August, it said. It did not mention any Turkish casualties.