Russian foreign Minister Sergei Lavrov and US Secretary of State John Kerry have stressed the need for further cooperation between Moscow and Washington to ensure a ceasefire in Syria.

In a phone conversation, the two sides also agreed to ramp up the campaign against terrorist groups in the war-torn country.

Meanwhile, Syrian officials said the Russian air force and Syrian military are preparing a joint operation to take Aleppo from rebels, and an opposition official said a cessation of hostility was on the verge of collapse.

The truce was brokered by the US and Russia in February to pave way for a resumption of peace talks. But it has been widely violated, with each side blaming the other for breaches.