The United States suspended Monday its cooperation with Russia on Syria, citing the failure of a ceasefire and Moscow's inability to end the violence.
"This is not a decision that was taken lightly," State Department spokesman John Kirby said in a statement, pointing to extensive negotiations that the US and Russia have conducted thus far in an effort to end the civil war.
"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."
A high-ranking source in the Russian Foreign Ministry told the Interfax news agency the US decision "evokes disappointment." The unidentified source said the ministry must further analyse the US decision before officially responding.
The US statement cites Russia and Syria's intensified military attacks against civilian areas, infrastructure and hospitals, including an attack on an aid convoy last month.
It said the US woud withdraw personnel that were to have worked to establish a Joint Implementation Center with the Russians, but would continue to use channels to "deconflict conterterrorism operations."
US Secretary of State John Kerry threatened last week to end US engagement with Russia on Syria, which further escalated tensions with Moscow over its involvement in the war-ravaged country.
Kerry had demanded Russia take "immediate steps" to end a military campaign on the rebel-held section of Aleppo.
The move indicated that "everybody's patience with Russias has run out," White House spokesman Josh Earnest said.
"What's clear is there is nothing more for the US and Russia to talk about with regards to trying to reach an agreement to end the violence inside of Syria, and that's tragic," Earnest said.
In Syria, at least three workers trying to repair a hospital in rebel-held eastern Aleppo were killed when it was hit by a third airstrike inside a week.
The airstrike killed another four people outside the M10 underground hospital in the besieged enclave, said Dr Abu al-Izz of the Syrian American Medical Society.
The repeated attacks on hospitals in rebel-held Aleppo, apparently by the Syrian and Russian air forces, have drawn condemnation from the United Nations and Western countries.
Al-Izz and Ibrahim al-Hajj of the White Helmets rescue organization in eastern Aleppo said the hospital had been hit by a "bunker-buster" bomb, which they said were the type dropped by Russian planes.
Russia intervened in Syria a year ago to prop up President Bashar al-Assad's crumbling army.
Its air campaign has allowed government forces to reclaim some territory from both rebels and the Islamic State extremist group, most notably imposing a siege on eastern Aleppo where some 250,000 to 300,000 civilians are now thought to be trapped.
Meanwhile, a key Syrian rebel group with close links to al-Qaeda said one of its senior leaders had been killed in an air strike by a US-led coalition.
The Fatah al-Sham Front, which until July was known as al-Nusra Front, described Ahmed Salameh, also known as Abu Faraj al-Masri, as a member of its "consultative council."
Salameh was one of two officials appearing alongside Fatah al-Sham's leader in a July video statement that announced the group's renaming and its apparent break from al-Qaeda, experts on the organization said at the time.
Pentagon spokesman Jeff Davis said an airstrike targeted a senior member of "core al-Qaeda" in Syria, and US forces were assessing the results.
The US has repeatedly targeted al-Nusra leaders as part of its wider air war in Syria, which is mainly focused on the terrorist group Islamic State.
The strikes on al-Nusra have been unpopular with the Syrian opposition, much of which sees the jihadists as key military allies on the ground regardless of ideological differences.
It has played a key role in major opposition offensives such as an assault in August that allowed rebels to break the siege of eastern Aleppo for several weeks.
Meanwhile, at least 21 Turkish-backed Syrian opposition rebels were killed while combing an area previously held by Islamic State north-east of Aleppo, a monitoring group said.
It is the largest death toll inflicted among the ranks of rebels backed by Turkey since they started their offensive against Islamic State near the Turkish-Syrian border on August 24.
By Anne K Walters and Weedah Hamzah
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