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 IS-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.
Local witnesses said at least some of the commandos spoke Arabic. There was no immediate comment from the US-led coalition.
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In the northeastern province of Hassakeh, a car bomb exploded Monday night in the predominantly Kurdish town of Qamishli, wounding several people, including the driver of the vehicle, state TV, the Observatory and a local official said.
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.
"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.
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