GENEVA — Russia and Iran criticized the U.N. chief’s decision to withdraw Tehran’s invitation to join this week’s peace conference on Syria, as delegates began to arrive in Switzerland today for the long-awaited talks that aim to end the Syrian civil war.

A last-minute U.N. invitation for Iran to participate in the so-called Geneva conference threw the entire meeting into doubt, forcing U.N. chief Ban Ki-moon to rescind his offer late Monday under intense U.S. pressure after Syria’s main Western-backed opposition group threatened to boycott.

After Ban withdrew the invitation, the opposition Syrian National Coalition said it would attend the talks aimed at ending Syria’s crippling three-year civil war, which has killed more than 130,000 and uprooted millions. The opposition said the conference should seek to establish a transitional government with full executive powers “in which killers and criminals do not participate.”

That cleared the way for the negotiations to open Wednesday as planned in the Swiss resort city of Montreux, with high-ranking delegations from the United States, Russia and close to 40 other countries attending. Face-to-face negotiations between the Syrian government and its opponents are to start Friday in Geneva.

Expectations for a breakthrough at the conference are low. The front lines of the war have been largely locked in place since March, and despite suffering their enormous losses, neither the government nor the opposition appears desperate enough for a deal to budge.

It’s also unclear how the opposition Coalition, a weak and fractured umbrella group with almost no sway over the most powerful rebel groups inside Syria, could enforce any agreement reached in Geneva.