Syria's main opposition umbrella group was meeting in the Saudi capital on Monday as Washington and Moscow worked to secure a ceasefire.

‘There is a meeting,’ Monzer Makhous, a spokesman for the High Negotiations Committee (HNC), told AFP in Riyadh.

The meeting is expected to continue for two or three days to discuss developments since the group decided to attend peace talks in Geneva last month, Makhous said.

He did not give further details.

World powers, which have been pushing for a halt to Syria's nearly five-year war, had hoped to see a truce take effect last Friday but have struggled to agree on the terms.

They proposed the truce as part of a plan that also included expanded humanitarian access, in a bid to pave the way for the United Nations-led peace negotiations to resume.

The talks collapsed earlier this month. They had been scheduled to resume this Thursday but UN Syria envoy Staffan de Mistura has acknowledged that date is no longer realistic.

At the weekend, the opposition said it would agree to a truce only if regime backers Moscow and Tehran halted their fire.

HNC chief Riad Hijab said any ceasefire must be reached ‘with international mediation and with guarantees obliging Russia, Iran and their sectarian militias and mercenaries to stop fighting’.

Russia, an architect of the proposed ceasefire, is conducting air strikes in support of President Bashar al-Assad's regime, which is also supported by Iran.

The HNC was formed in December when the main Syrian opposition and rebel factions came together in Riyadh for an unprecedented bid at unity, after months of Saudi mediation efforts.