Syrian rebels say they are suspending negotiations to join peace talks in Kazakhstan later this month because of what they say are repeated cease-fire violations by Syrian government forces.

A coalition of mostly moderate rebel groups, the Free Syrian Army, put out a statement Monday saying, “they respected the cease-fire across the whole of Syria … but the regime and its allies have not stopped shooting and have launched major and frequent violations.”

The rebels say any advance by the Syrian army violates the cease-fire and that they will consider the truce to be “null and void.”

Russia and Turkey, which are organizing the peace talks in Astana, have not yet commented on the rebel statement.

The cease-fire took effect late last week. But the rebels say Syrian fighters and their Hezbollah allies are still striving to recapture a rebel-held area near Damascus in which the city’s water supply is located.

Russia, which backs the Assad government, and Turkey, which supports the rebels, have organized the talks and also worked out the current cease-fire.