Russian warplanes are taking part in air strikes against insurgents to help repel a major attack on regime-held areas near the city of Hama, a Syrian military source said on Friday.
Rebel groups launched the attack on Tuesday and have captured at least 11 villages and towns, according to the Syrian Observatory for Human Rights (SOHR), a Britain-based organisation that reports on the war.
It marks the biggest attack by rebels in months. While regime leader Bashar al Assad still holds the military upper hand in the war, the rebel gains have shown the challenge facing the Syrian army and allied militia as they fight on numerous fronts.
"Air strikes have now started, and there is concentrated artillery firing against the armed groups and the headquarters of their leaders and supply lines, paving the way for the counter-attack," the military source said. "The Russians are, of course, participating in these raids."
Rebel groups were focusing their Friday attack on the village of Qomhana, about 8 km (5 miles), north of Hama city, SOHR Director Rami Abdulrahman said.
TRT World's Shamim Chowdhury has more from Gaziantep, on Turkey's border with Syria.
Fighting in Damascus
The assault near Hama coincides with clashes in the capital Damascus, where rebels and the army are fighting on the edge of the city centre in the Jobar district for a fifth day amid heavy bombardment, state media and SOHR said.
The government accused rebels of launching attacks in an attempt to "impact negotiations in Geneva," regime news agency SANA reported.
The fighting around Damascus has cut 300,000 people off from humanitarian aid and pauses in the fighting are needed to let aid convoys in, Jan Egeland, UN humanitarian advisor on Syria, said in an interview on Thursday.
The fighting has underscored the bleak prospects for the peace talks as the current round formally gets underway on Friday.
TRT World's Ediz Tiyansan has more on the talks from Geneva.
Battle to take Raqqa from Daesh
The fight against Daesh to recapture the Syrian city of Raqqa is likely to start in the coming days, French Defence Minister Jean-Yves Le Drian said on Friday.
"France has always said that Raqqa was a major objective," Le Drian said. "Today, one can say that Raqqa is encircled, that the battle for Raqqa will start in the coming days."
"It will be a very hard battle, but a battle that is going to be of utmost importance," he added.
Daesh is not part of the Geneva peace process, nor is it included in a fragile ceasefire which Turkey and Russia brokered in December that has allowed the Geneva process to proceed despite the fighting.