ASTANA (Reuters) - The start of Syrian crisis talks has been delayed by a day after the late arrival of Syrian rebel negotiators, who have decided to only send a group technical experts after threatening to boycott the meeting, sources and the delegation said.

U.N. mediator for Syria Staffan de Mistura attends a news conference after a meeting at the United Nations in Geneva, Switzerland, December 1, 2016. REUTERS/Denis Balibouse

U.N. special envoy Staffan de Mistura - who had also been invited and attended the previous Astana meeting in January - said he would not attend the talks and Turkey also sent a lower-level delegation than before.

The talks also involve Syrian government backers Russia and Iran.

Russia and Turkey, which backs the rebels, have sought to revive diplomacy towards ending the war since the Syrian government and its backers defeated the rebels in Aleppo in December, their biggest defeat of the conflict.

A new round of broader, U.N.-backed peace talks is due to begin in Geneva next week.

Syrian rebels had threatened to boycott the Astana talks, accusing Russia of failing to get Damascus to comply fully with a ceasefire or take any confidence-building steps.

The rebel delegation said in an emailed statement on Wednesday it would send a group of “technical experts” to discuss ways of reinforcing a ceasefire.

Two sources close to the talks said the rebel delegation had not arrived in Astana in time to start the meetings on Wednesday, which prompted the delay - announced officially without any explanation.

One of the sources also said the sides hoped that talks on Thursday would produce a joint document.

Kazakhstan, Moscow’s close political ally, said last week the negotiations would focus on consolidating the ceasefire.

Delegations of the Damascus government and the rebels who attended the previous round of Astana talks refused to negotiate directly with each other or sign any documents at the time.

Senior Russian negotiator Alexander Lavrentiev told reporters on Wednesday that delegations which had already arrived in Astana held a series of bilateral meetings ahead of the main round of talks.

“I would like to say that work is progressing well, there are of course some difficulties but they can be overcome,” he said without elaborating. “We will try to reach agreement on these issues by tomorrow.”

He also said de Mistura’s absence had been expected. Turkey has sent a lower-level delegation to the talks, compared with the previous round, because some senior diplomats were accompanying President Tayyip Erdogan on a foreign visit, Lavrentiev said.

The main Syrian opposition body said on Wednesday it wanted face-to-face negotiations with the Damascus government about a political transition at the Geneva talks.

Salim al-Muslit, spokesman of the High Negotiations Committee (HNC), also told Reuters the opposition was sticking to its position that President Bashar al-Assad can have no role in the transition, saying “the heavy price paid by the Syrian people” would have been wasted if he remained in power.