Syrian President Bashar al-Assad says his government is ready to negotiate on "everything" in proposed peace talks in Kazakhstan but says it's not yet clear who will represent the opposition and no date has been set.

Assad also said a ceasefire brokered by Turkey and Russia, his most powerful ally, ahead of the talks was being violated and the army's role was to recapture an area near Damascus where insurgents control the main water supply for the capital.

He made the remarks in comments to French media that were published by the Syrian state news agency SANA.

Asked if the government was ready to discuss Assad's position as president, he said "yes but my position is linked to the constitution".

"If they want to discuss this point they must discuss the constitution," he said.

He indicated that any constitutional matter must be put to a referendum, and it was up to the Syrian people to elect the president.