Syrian President Bashar al-Assad has said his government is ready to negotiate on "everything" in proposed peace talks with rebels later this month, Syrian news agency Sana reported on Monday.

The talks, brokered by Russia and Turkey, are scheduled to take place in Kazakhstan before the end of January, but last week opposition groups said they had frozen the process in light of continued government strikes across the country. 

In an interview with the French media published by Sana news agency, Assad said it was not yet clear who would represent the opposition and no date had been set yet. 

The Syrian President accused the rebels of violating nationwide truce several times. Assad also defended the Army's push to recapture a rebel-held valley near Damascus where the main water supply to the capital had been turned off.

Asked if he would be willing to step down as President — a demand the rebels have insisted upon throughout the conflict, Assad said "Yes, but my position is related to the Constitution".

"So, if they want to discuss this point, they have to discuss the Constitution, and it is not owned by the government or the President or by the opposition." 

The Syrian President said any constitutional matter must be put to a referendum, and it was up to the Syrian people to elect the President. 

Assad, in reference to the bombing of east Aleppo, called "every war bad".

"Every war is about the killing and destruction. But how can you liberate the civilians in those (rebel-held) areas from the terrorists? Is it better to leave them under their oppression, where their fate is defined by those terrorists by beheading, by killing, by everything but not having a state?

"…Is that the role of the state, just to keep and watch? You have to liberate, and this is the price some times (the death of civilians)," he said. 

Assad's position in the war has been significantly bolstered following the recapture of the northern city of Aleppo in December, which was the last rebel urban stronghold in the country. 

The UN has said the relentless Russian-backed bombardment of besieged rebel-controlled neighbourhoods, which appeared to target civilian infrastructure such as hospitals and bakeries, could amount to war crimes. 

Several rounds of talks brokered by the UN have so far failed to bring cessation to the almost six-year-long conflict.