UN leader Ban Ki-moon is battling to save a Syria peace conference after the Syrian opposition threatened to withdraw if Iran takes part.
Ban's spokesman said he was "dismayed" by Iran's refusal to back an international statement calling for a transitional government in Syria and the Syrian opposition's threat to boycott the talks.
"Iran, despite assurances provided orally to the secretary-general, has made a disappointing public statement that suggests Iran does not accept" a statement agreed by the major powers in 2012 as the basis for the peace conference, Martin Nesirky told reporters on Monday.
"Some key participants have conditioned their acceptance to the inclusion or exclusion of other delegations. There are some inevitable mutually exclusive positions. The secretary-general is currently urgently considering his options in light of the disappointing reaction of some participants.
"I can also tell you that we have of course been in close contact with the Russians and Americans today, over the weekend, and indeed for weeks and days before on precisely this topic and also with a variety of other leaders."
The Syrian National Coalition, a key player in the so-called Geneva II meeting, had issued a deadline of 19:00 GMT on Monday for Tehran's invitation to be rescinded. It is furious that the UN extended an invitation to Iran, which provides financial and military support to President Bashar al-Assad.
As the deadline passed, the Coalition said it would await an expected statement from Ban about Iran's attendance.
Al Jazeera's Anita McNaught, reporting from Istanbul, said: "The one party they [the UN] really needed to reassure find themselves embarrassed, compromised, undermined and betrayed.
"This [Iran] is such a polarising question. In the Syrian streets this doesn’t play well at all. It is something impossible politically for the Syrian opposition to navigate. All they’re asking for is clarity.”
The Geneva declaration of June 30, 2012 calls for the creation of a transitional governing body in Syria with full executive powers.
Assad has, however, rejected any suggestion he should stand down. He said on Sunday there was a "significant chance" he would make a candidacy bid.to run in June's presidential race.
"I see no reason why I shouldn't stand," he said. "If there is public opinion in favour of my candidacy, I will not hesitate for a second to run for election."