The main Syrian opposition bloc has said it will not attend peace talks scheduled to begin in Geneva on Friday, in another setback to diplomatic efforts to end the five-year-long war.

The High Negotiations Committee (HNC), an opposition platform created in Saudi Arabia last month, said its delegation would not attend the talks because they had not received convincing answers from the UN to a set of demands for their participation.

Their conditions include a lifting of sieges, a halt to air strikes and the release of wrongfully detained prisoners of conscience.

"For certain we will not head to Geneva and there will not be a delegation from the High Negotiations Committee tomorrow in Geneva," George Sabra, a member of HNC, was quoted as saying by the Reuters news agency on Thursday.

The already delayed talks have been mired in controversy and disagreements over who should attend representing the opposition.

Al Jazeera's James Bays, reporting from Geneva on Friday morning, said that a Syrian government delegation was expected to arrive in Geneva but no "actual negotiations" could take place in the absence of the main opposition groups.

"They cannot start negotiating because although there are some opposition players here, none of the main opposition groups - the list of opposition political players and fighting groups drawn up in Saudi Arabia - are here.

"They say they want their demands met before they fly to Geneva, so the talks process is starting [but] the actual negotiation is very much delayed and on hold."

He added: "Most diplomats think probably if there are some concessions in the right language it might be possible to persuade them [to attend the talks[ but it’s going to be a very difficult decision for the opposition members in Riyadh."

Syrians under siege 'left with no hope': UN official

On Thursday, UN Special Envoy Staffan de Mistura appeared in a video message directly addressing the Syrian people, saying that the talks were expected to start "in the next few days".

Earlier in the week, de Mistura said the negotiations were expected to last for six months and would push for a nationwide ceasefire for all parties, other than the Islamic State of Iraq and the Levant and al-Nusra Front armed groups.

The Syrian conflict has killed at least 250,000 people, according to the UN, and more than half of Syria's prewar population of 22.4 million has been internally displaced or fled abroad.