The United States is urging all parties invited to this week's Syrian peace talks in Geneva to show up without preconditions.
The talks on a ceasefire and setting up a transitional government in Syria were supposed to have started Monday. They have been put off until Friday because of squabbles over who should be invited.
U.N. Syria envoy Staffan de Mistura sent out invitations Tuesday, but did not disclose who was on the list.
But various media reports say it includes representatives of Saudi- and U.S.-backed moderate opposition groups, members of the Russian-backed opposition, and Syrian Foreign Minister Wakid al-Moallem.
It was unclear Wednesday exactly who is planning to come to Geneva. The U.S.-backed opposition insists it should be the only opposition delegation there. It demands Russian airstrikes in Syria stop and the Syrian government lift sieges that are strangling a number of towns.
Russia has insisted that Syrian Kurds should also be allowed to come while Turkey says no.
U.S. State Department spokesman Mark Toner said all sides need to come to the table without making demands ahead of the talks.
"We believe it should seize this opportunity to test the [Syrian] 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," Toner said.
Not invited to the talks are Islamic State and the al-Nusra Front, an al-Qaida-linked group fighting to topple the Syrian government.
While the United States says representatives of Syrian President Bashar al-Assad should attend the peace talks, it says Assad - a dictator who murders his own people - cannot be a part of Syria's future.
Moscow, Assad's strongest ally, says that decision is up to the Syrian people.