TEHRAN, IRAN (CBS) - Iran is criticizing the U.N. chief for rescinding his last-minute invitation to Tehran to attend the Syria peace talks, calling the diplomatic about-face "deplorable."
The talks are set to begin Wednesday in the Swiss luxury resort of Montreux, then move to Geneva.
Foreign Ministry spokeswoman Marzieh Afkham said Tuesday that Ban Ki-moon's decision to withdraw his offer to Iran shows the U.N. chief came "under some pressure."
"From our point of view," she said, "the withdrawal is deplorable."
Afkham said Iran would expect Ban to explain the "real reasons" for withdrawing the invitation.
Ban's gesture on Sunday put the talks in jeopardy, setting off a flurry of diplomatic activity aimed at salvaging them, as the Syrian opposition threatened to boycott the gathering and the U.S. pushed hard for the rescission of the offer to Iran, Syria's main regional ally.
With the invitation withdrawn, the main Western-backed opposition group, the Syrian National Coalition, said it would attend the talks aimed at ending Syria's ruinous three-year civil war.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."
The confusion surrounding the Iranian invitation underscored the tenuous nature of the diplomatic effort to end the bloody conflict, which has morphed from peaceful protests into a vicious civil war with outside powers backing rebels who are fighting not only the government but rival insurgents, as well.
The conference is set to begin 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.
Separately, Britain's Guardian newspaper reported Tuesday that Syrian regime officials "could face war crimes charges in the light of a huge cache of evidence smuggled out of the country showing the 'systematic killing' of about 11,000 detainees, according to three eminent international lawyers."
One told the Guardian the killings were "industrial scale."
The evidence included thousands of government photos and files on deaths of people in regime custody from March 2011 to last August, the Guardian said.
Russia's foreign minister said Tuesday that Ban's decision to rescind the invitation to Iran was a mistake but not a catastrophe.
Sergey Lavrov said Ban's decision would have a negative impact on the United Nations image.
"This story hasn't helped strengthen the U.N. authority," Lavrov said at a news conference, adding 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 Syria, shielding Syrian President Bashat Assad's regime from the United Nations sanctions and continuing to supply it with weapons throughout the civil war that has killed more than 130,000.
He 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.
Lavrov warned that spurning Iran would deepen division lines in the Islamic world and would have a negative impact on global efforts to fight terrorism.
"The absence of Iran isn't going to help strengthen the unity of the world's Muslims," he said.
Lavrov insisted Russia isn't supplying Syria with any weapons that are "banned by international agreements and could destabilize the situation in the region."
At the same time, Lavrov voiced hope for the success of the talks that would put the government and the opposition at the same table for the first time since the start of the conflict three years ago.
"There is no catastrophe, we will push for a dialogue between the Syrian parties without any preconditions," he said.
Ban, his spokesman said, made the invitation to Iran after Iranian officials assured him they supported the conclusion of a U.N. conference in 2012, known as Geneva-1, which called for a transitional administration to take over power in Syria, something neither Assad nor Tehran have been willing to embrace.
On Monday, Iranian officials made clear that they were not endorsing that conclusion as a basis for the talks.