GENEVA (AP) -- 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 on Tuesday 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.