BEIRUT — The invitations are sent and preparations are underway at the U.N.’s Palais des Nations in Geneva, where the first peace talks in two years on the conflict in Syria are to begin Friday.

But two days before the talks, it is unclear who will attend – or even if the U.N. special envoy to Syria will be able to move the needle on any of the thorny issues on the agenda to help end the war that has killed 250,000 people in the last five years.

In the chaotic run-up to the talks, the warring sides and their international backers have bickered over who should be present and what should be discussed, with some threatening to boycott if their conditions are not met.

The drama continued Wednesday with a major opposition bloc saying it would only join the talks if progress is made toward lifting sieges on blockaded towns in Syria and implementing U.N. Security Council resolutions on other humanitarian issues. The Saudi-backed bloc known as the Higher Negotiating Committee was meeting to make a final decision on whether to go to Geneva.

The U.S. on Wednesday called on the opposition to attend the talks.

“We believe it should seize this opportunity to test the regime’s willingness and intentions and expose before the entire world which parties are serious about a potential peaceful political transition in Syria and which are not,” State Department Spokesman Mark Toner said.

The wrangling has cast uncertainty on the talks, which already are generating very low expectations. The process is aimed at getting the sides to discuss implementing a national cease-fire and a political transition ending in elections.

U.N. special envoy to Syria Staffan de Mistura has sent invitations without making them public because of sensitivities surrounding participation.

The largest Kurdish group in Syria, the Democratic Union Party or PYD, is not invited; neither are the Islamic State group and the Nusra Front, two militant factions that control large parts of Syria.