Russia will back Assad in his push to reclaim Aleppo
Syrian and Russian forces are preparing a joint operation to retake Aleppo, the country's largest city, they announced yesterday.
Opposition groups, meanwhile, have warned that the fragile ceasefire is now on the verge of collapse.
Wael al-Halaki, Syria's prime minister, told a delegation of Russian legislators of plans to "liberate" Aleppo from "illegal armed groups".
While an offensive is thought unlikely to succeed, it would push more civilians towards Turkey as refugees.
The move comes a month after a promised draw-down of his forces by President Vladimir Putin, who declared that their mission had been accomplished.
He withdrew some Russian forces, but maintained an air base in Latakia and kept up strikes on Isil.
The deployment of the Russian air force to Syria last year helped tip the war in favour of President Bashar al-Assad as it bombed rebels supported by his enemies, including Saudi Arabia, Turkey and the United States.
The use of Russian firepower in the government offensive to retake Aleppo clouds the future of a fragile ceasefire that is currently holding across most of the country.
The Syrian opposition has warned that while the truce had largely held so far, despite numerous violations, it is about to disintegrate.
"Over the last 10 days, we have seen a very serious deterioration and the ceasefire is about to collapse," said Bassma Kodmani, a member of the high negotiations committee of the Syrian opposition.
Aleppo, formerly Syria's economic capital and the country's second city, has been divided since 2012 between zones held by rebel groups and areas still under government control.
Clashes around Aleppo killed at least 16 pro-regime fighters and 19 members of Al-Nusra Front, the al-Qaeda affiliate, and allied rebel groups within a 24-hour period, the London-based Syrian Observatory for Human Rights said yesterday. Free Syrian Army groups blamed the fighting on government violations.
"The air strikes are now roughly back to what they were," said Mohamed Rasheed, head of the media office with the Jaysh al-Nasr rebel group.
A Syrian military source said: "The battles are raging because … armed groups that were part of the (truce) joined Nusra in the attack."
The Observatory also reported fighting yesterday between government and rebel forces near the opposition-held town of Douma outside the capital, Damascus.
A ceasefire between the Assad regime and the rebels, brokered by Washington and Moscow, has been in place since February 27.
The truce does not include areas where Isil the Nusra Front are present.
Talks seeking to end the Syrian war are due to resume in Geneva on April 13.