Russian Foreign Minister Sergei Lavrov said on Friday that his country has observed positive signs in the Syria peace process and sees a meeting in the Kazakh capital next week as an important step toward establishing a framework for talks taking place in Geneva, according to Russian news agencies.

Lavrov added that Moscow was ready to do its part to move to a constructive dialogue with the United States.

The Syrian government forces intensified their attacks near Damascus and around 10 rebel groups who said they were suspending talks about planned peace negotiations this month.

The talks are due to take place in the Kazakh capital Astana in late January but the rebels said they were pulling out of discussions due to "violations" by Damascus of a four-day old truce.

The talks are being organized by Russia, which supports the Syrian regime, and Turkey and Iran, which back the rebels.

"As these violations are continuing, the rebel factions announce... the freezing of all discussion linked to the Astana negotiations," they said in a joint statement.

The rebels said they "respected the ceasefire across the whole of Syria... but the regime and its allies have not stopped shooting and have launched major and frequent violations, notably in the (rebel) regions of Wadi Barada and Eastern Ghouta", near Damascus, they said.

For the past two weeks, even before the start of a nationwide truce brokered by Ankara and Moscow, Syria's air force has launched almost daily bombing raids on Wadi Barada, some 15 kilometres from Damascus.

"Any (advance) on the ground goes against the (ceasefire) agreement and if things don't return to how they were before, the accord will be considered null and void," the rebel statement added.

On Monday, the Syrian army backed by air strikes and artillery fire advanced as it battles to capture the area, which is key to the capital's water supply, the Syrian Observatory for Human Rights said.

"Regime forces and fighters from Lebanon's Hezbollah group are advancing in the region and are now on the outskirts of Ain Al Fijeh, the primary water source in the area," said Rami Abdel Rahman, director of the monitoring group.

He said government troops and allied fighters were engaged in fierce clashes with rebels, including former Al Qaida affiliate Fateh Al Sham Front, a claim denied by opposition fighters.

Two civilians were shot dead by snipers and two other civilians were killed in regime bombardment of the town of Rastan in central Homs province, the Observatory added.

Government forces have surrounded Wadi Barada since mid-2015, but the siege was tightened in December as the army piled on the pressure.