The Syrian opposition has said it will not attend peace talks due to begin in Geneva tomorrow, derailing the first attempt in two years to hold negotiations aimed at ending the five-year-long war.

An opposition council convening in Saudi Arabia said its delegation would "certainly" not be in Geneva, saying it had not received convincing answers to its demands for goodwill steps including an end to air strikes and blockades.

The failure to get talks off the ground on time reflects the challenges facing peace-making as the conflict rages unabated on the ground.

The Syrian government is clawing back territory from rebels with military help from Iran and Russia.

It has said it is ready to attend the negotiations, which UN envoy Staffan de Mistura plans to hold in an indirect format.

Another opposition representative said the delegation might turn up if their demands were met in a day or two, but the chances of that appeared vanishingly slim.

The turn of events is a bitter blow to Mr de Mistura, whose office had issued a video message that he had sent to the Syrian people, in which he said the talks were expected to happen "in the next few days".

A spokeswoman for his office, speaking before the opposition statement, said the talks would begin tomorrow as scheduled.

George Sabra, a member of the opposition High Negotiations Committee, said: "For certain we will not head to Geneva and there will not be a delegation from the High Negotiations Committee tomorrow in Geneva."

Before agreeing to talks, the HNC had been seeking UN guarantees of steps including a halt to attacks on civilian areas, a release of detainees, and a lifting of blockades.

The measures were mentioned in a Security Council resolution approved last month that endorsed the peace process for Syria.

Mr Sabra said a response from Mr de Mistura was "unfortunately still ink on paper".

"We are not certain that the opportunity is historic," he told Arabic news channel Arabiya al-Hadath.

Another HNC official said the opposition could attend if their demands were met "within two, three or four days."

"Tomorrow will probably the start will be with those who attend but it has no value," Monzer Makhous told Al-Hadath.

The talks were meant to start in Geneva on Monday but the UN has pushed them back to tomorrow to allow more time to resolve problems, including a dispute over which groups should be invited to negotiate with the government.

The exclusion of a powerful Kurdish faction that controls wide areas of northern Syria has triggered a boycott by some of the invitees.

Turkey had opposed the PYD's participation on the ground it views it as a terrorist group.

The United States, whose Secretary of State John Kerry is among those pushing for negotiations to start tomorrow, urged the opposition to seize the "historic opportunity" and enter talks without preconditions to end the war, which has also displaced more than 11 million people.

Diplomacy has so far had little impact on the conflict, which has spawned a refugee crisis in neighbouring states and Europe.

Mr de Mistura is the third international envoy for Syria.

His two predecessors - Kofi Annan and Lakhdar Brahimi – both quit.