A Syrian opposition umbrella group said on Saturday it welcomed a complex truce deal brokered by Moscow and Washington provided it was honoured by the regime.

"We hope this will be the beginning of the end of the civilians' ordeal," leading High Negotiations Committee member Bassma Kodmani said.

"We welcome the deal if it is going to be enforced," she told the BBC, and said the HNC was "absolutely in favour" of a cessation of hostilities.

In comments to AFP, she said the HNC "cautiously welcomed" the agreement.

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov said that the truce, reached in Geneva late Friday, would come into force on Monday, the first day of the Muslim holiday of Eid al-Adha.

The two powers back opposing sides in the conflict, with Moscow supporting the regime of President Bashar al-Assad and Washington backing a coalition of rebel groups it regards as moderate.

But if Russia is able to pressure Assad to respect the ceasefire for a week, Moscow and Washington will set up a joint coordination unit and begin joint air strikes against agreed "terrorist" targets.

The two governments regard both the Islamic State group and its jihadist rival, former Al-Qaeda affiliate the Fateh al-Sham Front, as "terrorist" organisations and both are excluded from the ceasefire plan.

But Islamist rebel groups have been closely allied with Fateh al-Sham in fighting pro-government forces in northern Syria, drawing repeated rebukes from Moscow.

Kodmani said the rebels would break ranks with the jihadists if the truce deal held.

"The moderate groups will reorganise and distance themselves from the radical groups. We will do our part," she said.