0848 GMT June 26 2017
Assad also said an ongoing cease-fire brokered by Turkey and Russia last month was being violated and the army would recapture all of Syria including a terrorist-held area near Damascus where a vital water supply had been bombed out of service, Reuters reported.
He made the remarks in comments to French media.
Assad said the government delegation was ready to go to the Kazakh capital of Astana "when the time of the conference is set".
The talks are expected to take place later this month.
"We are ready to negotiate about everything," he said. Asked if that included his position as president, Assad 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 new constitution must be put to a referendum, and it was up to the Syrian people to elect the president.
Assad said: "Who will be there from the other side? We do not yet know. Will it be a real Syrian opposition?"
Dismissing groups he said were backed by Saudi Arabia, France and Britain, Assad said discussion of "Syrian issues" must be by Syrian groups.
The Syrian Army has been trying to recapture the Wadi Barada valley where the capital's main water source is located.
Assad said the Wadi Barada area was held by the Nusra Front terrorist group which is not covered by the cease-fire. "The terrorists occupy the main water source for Damascus, denying more than five million civilians water for more than three weeks," he said.
"The Syrian Army's role is to liberate that area," he said.
Asked if the government planned to recapture the Daesh-held city of Raqqa, Assad said it was the Syrian Army's role to liberate "every inch" of Syrian land and all Syria should be under state authority.
"But the question is related to when, and our priorities. This is a military matter linked to military planning and priorities," he added.