US threatens to stop cooperating with Russia on Syria

US Secretary of State John Kerry told his Russian counterpart, Sergei Lavrov, on Wednesday that Washington would suspend bilateral cooperation with Moscow on Syria unless Russia took immediate action to end the offensive in Aleppo and restore a collapsed ceasefire.

Kerry spoke with Lavrov by telephone, EFE news quoted the State Department as saying in a statement.

Kerry "informed the Foreign Minister that the United States is making preparations to suspend US-Russia bilateral engagement on Syria," State Department spokesman John Kirby was quoted as saying.

Cooperation will be halted "unless Russia takes immediate steps to end the assault on Aleppo and restore the cessation of hostilities" that had been agreed by Washington and Moscow but which ended more than a week ago, the statement added.

An end to cooperation would mean suspending "establishment of the Joint Implementation Centre," a mechanism for planned joint US-Russian strikes in Syria on Islamic State and Nusra Front targets once the truce has held up and humanitarian aid has flowed unimpeded for a period of seven days.

Kerry and Lavrov tried to reach agreement on a renewed ceasefire during last week's UN General Assembly meeting, but the talks broke down due to the countries' sharp differences and Syrian troops' fresh offensive on rebel-held areas of Aleppo, an operation backed by Russian warplanes.

In his conversation Wednesday with Lavrov, Kerry "expressed grave concern over the deteriorating situation in Syria, particularly for continued Russian and Syrian regime attacks on hospitals, the water supply network, and other civilian infrastructure in Aleppo," Kirby said in the statement.

"The secretary made clear that the United States and its partners hold Russia responsible for the situation, including the use of incendiary and bunker buster bombs in an urban environment, a drastic escalation that puts civilians at great risk," the statement added.

Kerry "stressed that the burden remains on Russia to stop this assault and allow humanitarian access to Aleppo and other areas in need," the spokesman added.