BEIRUT: Key Syrian rebel group Ahrar al-Sham said on Wednesday it will not take part in peace talks between the government and opposition factions in the Kazakh capital next week.

The group decided not to participate in the negotiations in Astana that start on Monday due to "the lack of implementation of the cease-fire" in force since December 30 and ongoing Russian airstrikes over Syria, it said in a statement.

Ahrar al-Sham was among rebel groups that signed the cease-fire deal brokered by government supporter Russia and rebel backer Turkey last month.

The truce has largely held across Syria although fighting has persisted in some areas, allowing Russia, Turkey and government supporter Iran to organize the peace talks in Astana.

Ahrar al-Sham said "the regime's offensive against our people in Wadi Barada,"an area 15 kilometers (10 miles) northwest of Damascus that is the capital's main source of water, was among the reasons it would not attend the talks.

Assad's forces have pressed an assault to retake the area from rebels after mains supplies were cut last month, leaving 5.5 million people in Damascus and its suburbs without water.

Ahrar al-Sham said however that it was giving its support to other rebel groups represented at the Astana talks.

Mohammad Alloush, a prominent figure of the Jaish al-Islam (Army of Islam) faction, will in Astana head a "military delegation" of around eight people, backed by nine legal and political advisors from the High Negotiations Committee umbrella group.

Russia started airstrikes in support of Assad's government in 2015.