Iran's president will miss a meeting of the UN Security Council in New York on Saturday because the US delayed issuing visas, Iranian officials say.
Mr Ahmadinejad said he wanted to address the Security Council
Mahmoud Ahmadinejad had planned to attend the meeting, at which a vote on imposing fresh sanctions over Iran's nuclear programme is possible.
But a spokesman for the Iranian mission told the BBC visas for the president's air crew were issued too late.
Talks on a resolution imposing more sanctions are continuing in New York.
"American authorities have effectively prevented President Ahmadinejad from attending the UN Security Council meeting," foreign ministry spokesman Mohammed Ali Hosseini said on Iranian state television.
The Iranian leader had wanted to address the Security Council before it holds any vote on further sanctions against Tehran for refusing to suspend uranium enrichment.
In December, the Security Council imposed limited sanctions on the country over its refusal to halt the programme, which Iran insists is purely peaceful in purpose.
The sanctions, agreed on last week by the six powers, would block Iranian arms exports and freeze the assets of individuals and companies involved in Iran's nuclear and missile programs.
It is still unclear whether the vote will take place on Saturday as last-minute negotiations continue at the UN headquarters.
Britain, France and the US want a Saturday vote but Qatar, Indonesia and South Africa, also members of the Security Council, have concerns.
The Iranian spokesman told the BBC that instead of the president, Iran's foreign minister and his deputy hoped to attend, travelling on a commercial flight.
The US ambassador, Alex Wolff, said that looked like an excuse and that all visas properly filled in had been granted.
A spokesman for the US mission to the UN also pointed out that a definitive date for any Security Council vote has not yet been scheduled.
"How can they say they are too late to come if the Security Council has not given a date and time for the event?" said Richard Grenell.
Deputy State Department spokesman Tom Casey said any accusations that visa issues were behind Mr Ahmadinejad's decision not to attend were false.
He said the Iranian leader was "unwilling to stand before the Council and take the heat for his continued defiance of the international community".
The US embassy in Berne, Switzerland, said that by the end of the day it had issued a total of some 70 visas for Mr Ahmadinejad and his entourage.
The BBC's Laura Trevelyan, in New York, says that if Mr Ahmadinejad is not present, the occasion will be considerably less dramatic.