The United Nations on Tuesday strongly defended Iran's exclusion from Syria peace talks after criticism from Russia and the Tehran government.
Iran failed to come up with a promised written statement on the Syria conflict and so UN leader Ban Ki-moon was forced to act, UN deputy spokesman Farhan Haq said.
Haq also said Ban was anxious that countries attending the talks, which start in Montreux, Switzerland, on Wednesday, were negotiating "in good faith."
Ban rescinded an invitation to Iran on Monday just 24 hours after announcing that the ally of President Bashar al-Assad would be at the talks.
The Syrian opposition had threatened to boycott the event if Iran was present and the United States insisted that Iran had to agree to an international communique calling for a transitional government in Syria.
UN officials said Ban spent several days negotiating with Iran's Foreign Minister Javad Zarif on publicly supporting the 2012 Geneva communique which calls for the transitional government in a bid to end Syria's three-year-old war.
"There was an oral understanding that the secretary general had been led to believe would be followed by an actual written understanding," Haq told reporters to explain the invitation to Iran.
"In fact the opposite is what happened, that Iran stated the same positions that it had held previously. And that is why he expressed his disappointment at Iran's decision and took his decision to disinvite them," the spokesman added.
Ban was in contact with the United States and other key parties during the talks, Haq added.
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Zarif blasted the UN decision and called on Ban to "provide the real reasons for the withdrawal."
"This behavior is beneath the dignity of the UN's secretary general," the minister was quoted as saying by Iranian media.
Russia's Foreign Minister Sergei Lavrov called Tehran's exclusion a "mistake."
Ban has insisted the aim of Wednesday's conference, to be followed by direct negotiations between Assad and opposition representatives in Geneva from Friday, is to implement the 2012 Geneva communique.
Iran has repeatedly refused to sign up to the Geneva document which was agreed by Russia, a strategic Assad ally, the United States, which backs the opposition, and other key powers.
"It doesn't really make sense to have someone participate in discussions if they are doing it without understanding what the basis of the discussions are," Haq said.
"Although the various parties come in with different understandings of the situation, different understandings of who to support and why, what we are doing is relying on them to act and negotiate in good faith," he added.
The makeup of the Syrian opposition delegation is uncertain after a key bloc broke away from the Syrian National Coalition on Monday in protest at its participation in the talks.
"We had wanted a broadly representative delegation, we will see tomorrow who is there to speak and participate," said Haq.
"But we continue to be hopeful that the delegation that is present in Montreux ... will be broadly representative of the Syrian opposition," added the spokesman.