An AFP reporter at peace talks in the Kazakh capital Astana saw the heads of the delegations, representing the three countries sponsoring the negotiations, sign the document.
However a member of the rebel delegation left the room, shouting against regime ally Iran, the AFP reporter saw. The Syrian government and rebel delegations are not signatories.
The Kremlin has been touting a plan to create safe zones in Syria that is aimed to "further pacification and cessation of hostilities."
"As a result the guarantor countries agreed to sign a memorandum on the creation of de-escalation zones in Syria."
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An Arabic-language version of the Russian draft 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.
But issues including which countries could police any safe zones remain unclear.
Russian President Vladimir Putin said yesterday that ways to monitor the zones would be an issue for separate talks.
Syrian rebels said earlier today that they had resumed participation in the talks after having suspended their involvement a day earlier over air strikes against civilians.
After talks with Turkey counterpart Recep Tayyip Erdogan yesterday, Russian President Vladimir Putin said the safe zones were meant to lead to "further pacification and cessation of hostilities."
The Kremlin's plan echoes calls by US leader Donald Trump to establish safe zones in Syria.
Putin said yesterday 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 today that Moscow's plan to set up these zones in Syria would "50 per cent" 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.