Russia's plan for "de-escalation" zones was backed by Iran at talks in Kazakhstan. Both countries support Syrian president Bashar al-Assad.
Turkey, which supports rebel groups, also agreed to act as a guarantor.
But some delegates from the rebel forces angrily rejected the plan.
As the three countries were invited to sign the document on Thursday in the Kazakh capital, Astana, rebel delegates angrily shouted that they did not accept it and walked out.