Officials said a decision rested on investigations into whether Mr Ahmadi-Nejad was involved in the 1979 US embassy hostage crisis and the killing of an Iranian-Kurdish dissident leader in Vienna in 1989. Iran denies his involvement in either event.

A top Iranian official confirmed Thursday that Mr Ahmadi-Nejad, who took office on Wednesday, planned to address the UN Millennium Summit and its annual General Assembly next month. Ahmadi-Nejad’s visa application was submitted on Thursday, the Iranian official said.

The trip would be “mutually beneficial to the US and Iran”, the official added.

A White House official said that visa applications were confidential under US law and therefore he could not comment on the outcome. Asked if the president’s alleged involvement in the 444-day-long embassy hostage crisis, if proven, would be sufficient reason to deny him a visa, the US official replied: “That is something we are looking at.”

A US official who asked to remain anonymous said agencies were examining whether there was sufficient evidence to deny a visa and how this would be justified under international law.

Stephane Dujarric, UN spokesman, said: “The host country agreement calls on the US not to impose any impediment to the travel to the UN of any representative of a member state on official business.”

Yasser Arafat, Palestine Liberation Organisation chairman, was denied entry in 1988. He addressed the UN in Geneva. Mohammad Khatami, Iran's previous president, spoke at the UN several times, most notably just weeks after the September 2001 attacks, which he condemned.

Former US diplomats allege they recognise Mr Ahmadi-Nejad as one of their captors, but the Central Intelligence Agency has found no confirmation of this.