Syria's main opposition body has agreed to travel to Geneva, where peace talks began today, following a phone call from the US Secretary of State and after receiving guarantees that their demands would be met, its spokesman said today.

The High Negotiations Committee (HNC) has repeatedly said the Syrian government and its allies must first halt bombing and lift blockades of besieged areas before it will join the talks, which have been delayed by four days, and signalled it would still leave the process if these measures are not implemented.

"Guarantees came that (United Nations) Resolution 2254 would be implemented in full...so we will go to Geneva to discuss humanitarian issues," spokesman Salim al-Muslat said.

"The HNC will go to Geneva tomorrow to discuss these humanitarian issues which will pave the way into the political process of negotiations," Mr al-Muslat said.

After receiving invitations to the talks this week, the Saudi-backed HNC wrote to the UN secretary general requesting clarification on the implementation of steps outlined in a December UN Security Council resolution that endorsed the peace process.

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

The decision to travel from Riyadh, where the HNC has been meeting to debate their attendance, came after a telephone call with US Secretary of State John Kerry and after receiving an answer from the UN to their letter.

"We have agreed on a certain time period, which I cannot disclose now," Mr al-Muslat said.

"Perhaps the coming days will show a serious approach to this issue. If there is then we will continue. If we don't see a serious approach then there will be a different decision," added Mr al-Muslat.

The Syrian government's delegation arrived today at the talks that UN envoy Staffan de Mistura hopes to convene in an indirect format with the aim of ending the five-year-old war that has killed 250,000 people.