The United Nations top envoy for Syria will announce before a ceasefire deadline at midnight Friday possible plans to resume peace talks for the conflict-ravaged country, he told reporters.

"Tomorrow is going to be a very important, I will say a crucial day," Staffan de Mistura said at the UN's European headquarters in Geneva.

He promised to meet with journalists around 2200 GMT Friday "to assess where we are and indicate also the information regarding the resumption of Geneva talks."

Before that can happen though, Syria's warring parties have until midday Damascus time (1000 GMT) to sign up for a ceasefire deal brokered by Moscow and Washington.

After that the 17-nation group backing Syria's peace process will meet in Geneva to approve the text before the 15 members of the Security Council are expected to adopt it, paving the way for the "cessation of hostilities" between President Bashar al-Assad's regime and non-jihadist rebel forces to begin by midnight Damascus time (2200 GMT).

De Mistura said he would brief the Security Council at 2000 GMT by video link from Geneva, before addressing the media in Geneva.

The ceasefire deal -- which excludes the Islamic State (IS) group and other Sunni extremists -- marks the biggest diplomatic push yet to help end the five-year conflict in Syria which has claimed more than 270,000 lives and displaced more than half of the population.

The proposed truce is part of a plan announced by top diplomats in Munich earlier this month that also included expanded humanitarian access.

"Both deliverables are moving, and moving in the right direction," de Mistura said, describing the work done since the Munich meeting as "momentous".

But while humanitarian access has made dramatic progress, with convoys reaching six besieged areas and delivering aid to 110,000 people, the cessation of hostilities failed to take hold last week as initially planned.

If it does kick off Saturday, that could pave the way for UN-led peace negotiations to resume after they collapsed in Geneva earlier this month.