U.N. diplomats are considering a Security Council resolution endorsing a U.S. and Russian backed cease-fire in Syria, which U.S. Secretary of State John Kerry says is "the one way that we can end this war."
Syria's warring parties have until midday Friday to say if they are participating in the cessation of hostilities that is due to begin 12 hours later.
U.S. and Russian officials are holding further discussions Thursday on implementing the cease-fire, and Russia's Deputy Foreign Minister Mikhail Bogdanov reiterated that Russia is not discussing any alternatives to the plan.
The Syrian government has said it will take part, but will continue attacking Islamic State and al-Qaida-linked terrorists, which are excluded from the cease-fire.
The main opposition High Committee for Negotiations (HNC) gave a reserved endorsement for the cease-fire on Wednesday, saying it will participate for two weeks to determine the commitment of the other side.