The Syrian opposition is ­awaiting clarification on key points from the United ­Nations before deciding whether to attend peace negotiations planned for Friday.

The Syrian government has already agreed to join the talks that UN envoy Staffan de ­Mistura hopes to convene in an indirect format in Geneva with the aim of ending the 5-year-old war that has killed 250,000 people.

But a Saudi-backed opposition council that includes armed and political opponents of President Bashar al-Assad has yet to decide whether to go. It viewed its invitation "positively," but said it had requested clarifications from UN Secretary General Ban Ki-moon.

The High Negotiations Committee (HNC) was ­meeting in Riyadh for a second day on Wednesday to decide its stance.

"We sent the questions. We are awaiting the response," said an opposition source familiar with the proceedings in Riyadh.

The source said their most important question covered the implementation of steps ­outlined in a UN Security Council resolution that ­endorsed the peace process.

These include the lifting of blockades on besieged areas, a halt to attacks on civilian areas, and a release of arbitrarily detained people.

Opposition officials had repeatedly said they could not agree to negotiations before such steps were taken by the government and its allies to show goodwill.

French Foreign Minister Laurent Fabius said he had spoken to HNC coordinator Riad Hijab and understood they would attend.

"I spoke to Mr Hijab ... he will respond to De Mistura and Ban Ki-moon," Fabius told France Culture radio.

"If I understand their position, they say yes to negotiations."

Diplomacy has so far failed to resolve the conflict that has forced millions from their homes, spawning a refugee crisis in neighboring states and Europe.

De Mistura is the third ­international envoy for Syria.

His two predecessors, Kofi ­Annan and Lakhdar Brahimi, both quit.

The war has continued unabated as De Mistura tries to launch the peace process endorsed by the Security Council last month.

The Syrian government, aided by Russian airstrikes and allied militia including Iranian forces, is gaining ground against rebels in western Syria, this week capturing the town of Sheikh Maskin near the Jordanian border.

Russian airstrikes that began on September 30 have tilted the war Assad's way after major setbacks earlier in 2015 brought rebel groups close to the coastal heartland of his Alawite sect.