In this Saturday, Jan. 18, 2014 citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, a Syrian man, left, runs next of a burned truck that was attacked by an explosive barrel thrown from a Syrian forces helicopter, in Aleppo, Syria. (AP Photo/Aleppo Media Center, AMC)

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 on Tuesday for the long-awaited talks.

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. 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 conference to open Wednesday as planned n 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 — the first of the uprising — are to start Friday in Geneva.

But Russia and Iran, Syrian President Bashar Assad's two closest allies, criticized the U.N. over the decision to exclude Tehran.

In Moscow, Russian Foreign Minister Sergey Lavrov said Ban's decision to rescind Iran's invitation was a mistake, but that Moscow would try to make the negotiations work.

"There is no catastrophe, we will push for a dialogue between the Syrian parties without any preconditions," Lavrov said at a news conference.

At the same time, Lavrov took a jab at Ban, saying his decision on Iran "hasn't helped strengthen the U.N. authority," and that recalling the offer looked "unseemly."

The controversy over Iran's participation in the talks reflected deep differences over Syria between the United States and Russia, which has been a key ally of Damascus, shielding Syrian President Bashar Assad's regime from U.N. sanctions and continuing to supply it with weapons throughout the civil war that has killed more than 130,000.

Lavrov reaffirmed Russia's stance that the presence of Iran was essential for the success of the talks.

Iran has been Assad's main regional ally, supporting his regime with advisers, money and materiel since the uprising began in 2011.

In Tehran, Iran's Foreign Ministry sharply criticized Ban's diplomatic about-face.

"From our point of view, the withdrawal is deplorable," ministry spokeswoman Marzieh Afkham said, adding that the U.N. chief only did so under immense pressure.

Afkham said that Iran expects Ban Ki-moon will explain the "real reasons" for withdrawing the invitation.

News from © The Associated Press, 2014