"The rebel delegation is suspending the meetings because of the violent air strikes on civilians. The suspension will continue until shelling stops across all Syria," a rebel source in the Kazakh capital Astana told AFP.
Syrian government and rebel delegations gathered on Wednesday for the start of a fourth round of talks sponsored by regime backers Russia and Iran and opposition supporter Turkey.
The latest round of negotiations began with a series of bilateral meetings and was set to focus on a Russian plan to establish "de-escalation zones" around the war-torn country.
A source close to the opposition provided AFP with an Arabic-language version of a proposal drafted by Russia, which an opposition official confirmed was being discussed on Wednesday.
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It calls for the creation of "de-escalation zones" in rebel-held territory in the northwestern province of Idlib, in parts of Homs province in the centre, in the south, and in the opposition enclave of Eastern Ghouta near Damascus.
The aim is to "put an immediate end to the violence" and "provide the conditions for the safe, voluntary return of refugees".
The designated zones would also see the immediate delivery of relief supplies and medical assistance.
According to the draft, "security zones" would be created around them, with checkpoints and monitoring centres to be manned by government troops and rebel fighters.
Military units from unspecified "observer countries" could also be deployed.
The document named Turkey, Iran and Russia as guarantors of the agreement and pledged that they would create a "joint working group" within five days of the document being signed by the warring parties.