However, the upcoming talks, brokered by Ankara and Moscow, are still in doubt as Syrian opposition groups have yet to confirm their participation.
Syrian activists meanwhile reported what appeared to be a US-led special forces raid on Islamic State militants in eastern Syria.
Omar Abou Leila, who runs Deir Ezzor 24, said four helicopters landed in the desert between the ISIS-held cities of Deir el-Zour and Raqqa on Sunday. Commandos set up checkpoints and intercepted a vehicle carrying several Islamic State militants, killing all of them and flying off with the bodies, he said.
"It's an operation that apparently targeted an important figure," Abou said, from Germany, where he is based. Deir Ezzor 24 is one of several locally staffed underground groups reporting from ISIS-held territory.
The Britain-based Syrian Observatory for Human Rights, another activist-run group, said 25 militants were killed in the ambush.
Local witnesses said at least some of the commandos spoke Arabic. There was no immediate comment from the US-led coalition.
Speaking to French reporters at his Damascus palace, Assad defended his troops' deadly bombardment of eastern Aleppo, saying the alternative would have been to leave the city's civilians to the mercy of "terrorists" — a term the government uses for all rebels.
Assad questioned the credibility of Syrian opposition groups backed by the West and Saudi Arabia, which make up the bulk of the armed and political opposition to his rule.
"There's no limit to negotiations," Assad said, in remarks carried by Syrian state media. "But who is going to be there from the other side, we don't know yet. The viability of the conference depends on that."
Past Syrian peace talks have run aground on the question of Assad's future and whether he is to continue as president, with the opposition insisting his departure is a precondition for any reforms.
Assad said the matter could only be resolved through a constitutional referendum.
"If they want to discuss this point, they need to discuss the constitution. You need a referendum for every [constitutional amendment]. This is one of the points that could be discussed in the meeting" in Kazakhstan, he said.
The talks are scheduled to begin in the Kazakh capital of Astana on January 23. They follow a lengthy rapprochement between Russia, a key backer of Assad, and Turkey, a main sponsor of the opposition, that culminated in a cease-fire agreement that came into force on December 30, but which has already started to erode.
Russian officials have suggested the US could be invited to the talks at a later date.
The Obama administration has been at odds with Russia over how to resolve Syria's conflict. Incoming President Donald Trump has indicated he might distance the US from Syria's rebels, bringing Washington in closer alignment with Moscow.
Trump tweeted on Saturday that he would strive to build good relations with Russia, and "perhaps, work together to solve many of the great and pressing problems and issues of the WORLD!"
Asked about the comments, Assad said warmer relations between Washington and Moscow "reflects positively on the Syrian conflict".
As for Aleppo, Assad said the government forces were forced "to liberate" the city.
"There is a price, sometimes, but at the end the people are liberated from the terrorists," he said.
Once Syria's largest city and industrial hub, Aleppo has been devastated by nearly six years of war. Rebels took control of its eastern districts in 2012, before surrendering it to government authority last month.
The UN said the government's relentless military campaign, which displaced tens of thousands of civilians, could have violated the laws of war.