Syria's main opposition bloc has backed planned peace talks in Kazakhstan's capital later this month between the Syrian government and rebel groups.

A nationwide ceasefire began in Syria on December 30 to pave the way for new peace talks, which Russia hopes to convene in Astana on January 23 with Turkish and Iranian support.

After a two-day meeting in Riyadh, Saudi Arabia, the High Negotiations Committee (HNC) said on Saturday it would extend its support to an anti-government military delegation attending the talks.

"Concerning the forthcoming meeting in Astana, the (High Negotiations) Committee stresses its support to the military delegation ... and expresses hope that the meeting would reinforce the truce," the HNC said.

It added the Astana summit "paves the way for political talks" in Geneva in early February, hosted by the United Nations.

Also on Saturday, Turkey's foreign minister Mevlut Cavusoglu said Ankara and Moscow had decided to invite the United States to the Astana talks.

Cavusoglu also reiterated that Turkey remained opposed to the inclusion of the People's Protection Units (YPG), the Syrian Kurdish armed group, in the meeting.

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Syria has been ravaged by violence since widespread protests in March 2011 calling for President Bashar al-Assad's ouster.

More than 310,000 people have been killed and over half the population has been forced to flee.

Diplomatic efforts to end the conflict have thus far failed, but Moscow and Ankara are hoping that this month's talks will lead to a political solution.

Outbreaks of violence

The ceasefire deal appeared increasingly strained on Saturday, with outbreaks of violence in northwest Syria and near the capital, Damascus.

Fresh raids on Saturday in the town of Maarat Masrin in the northwest province of Idlib killed eight people, most of them civilians, the Syrian Observatory for Human Rights said.

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A day earlier, three civilians - including a child - were killed in strikes on the nearby town of Orum al-Joz, Rami Abdel Rahman, the head of the UK-based monitoring group tracking developments in Syria's conflict through a network of contacts on the ground, said.

Idlib province is controlled by a rebel alliance led by Jabhat Fateh al-Sham, which changed its name from al-Nusra Front after breaking ties with al-Qaeda last year.

Like the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, Jabhat Fateh al-Sham is excluded from the truce deal.

New clashes also broke out Saturday in Wadi Barada, the main source of water for Damascus.

Water supplies from the area to around 5.5 million people in the capital and its outskirts have been cut since December 22 because of fighting.

Rebels and government troops had reached a local agreement on Friday so that water access could be restored, but the Observatory reported a resumption of violence on Saturday.

"Regime forces and [Lebanese movement] Hezbollah violated the agreement" by battering a town in Wadi Barada with rocket fire, Abdel Rahman told AFP news agency.

"They took advantage of the halt in military activities there to advance and flex their muscles," he said.