The rebels on Wednesday said they were temporarily pulling out of the talks -- sponsored by regime backers Russia and Iran and opposition supporter Turkey -- over air strikes against civilians.
"The delegation of the military revolutionary factions has decided to halt the suspension of its participation in negotiations after receiving new guarantees from the guarantor countries," the Syrian opposition said in a statement.
An AFP reporter on Thursday saw members of the rebel delegation at the Astana hotel where the talks were set to resume.
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The fourth round of Astana talks revolves around a Russian plan to establish "de-escalation zones".
An Arabic-language version of the Russian proposal seen by AFP 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" as well as the immediate delivery of relief supplies and medical aid, the document said.
After talks with Turkey counterpart Recep Tayyip Erdogan on Wednesday, Russian President Vladimir Putin said the safe zones were meant to lead to "further pacification and cessation of hostilities."
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He also said the proposed zones would also be no-fly areas if fighting on the ground there stopped entirely.
The Kremlin's plan echoes calls by US leader Donald Trump to establish safe zones in Syria.
Putin said Wednesday that "as far I could tell" the US leader broadly supported the idea in a phone call they held on Tuesday.
Erdogan said in comments published Thursday that Moscow's plan to set up these zones in Syria would "50 percent" solve the six-year conflict.
Damascus supports the Russian plan, Syrian state news agency SANA reported.
More than 320,000 people have been killed in Syria since the country's war began with anti-government protests in March 2011.
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