The US adopted a harsher tone toward Iran's proposed UN ambassador on Tuesday, calling Tehran's choice of Hamid Abutalebi "unacceptable" and tying him to the 1979-1981 US hostage crisis in Tehran.

While it did not detail what the veteran diplomat may have done during the period, when radical Iranian students seized the US Embassy and held 52 US hostages for 444 days, the State Department for the first time linked the US decision not to issue him a visa to those events.

"He himself has said he was involved and, given his role in the events of 1979, which clearly matter profoundly to the American people, it would be unacceptable for the United States to grant this visa," State Department spokeswoman Jen Psaki told reporters at her daily briefing.

Abutalebi has said that he acted only as a translator.

Previously US spokespeople used softer language, saying the choice was not "viable."

The US on Friday said it had told Iran it would not give Abutalebi a visa.

US officials privately say they hope Iran might quietly drop the issue and name a new envoy.

However, Iran on Monday asked for a special meeting of a UN committee on the US decision, calling it a dangerous precedent that could harm international diplomacy.

Officials, diplomats and academics could not recall past cases of the US denying a UN ambassador's visa.

Under a 1947 "headquarters agreement," the US is generally required to allow access to the United Nations for foreign diplomats.

Washington says it can deny visas for the reasons of "security, terrorism, and foreign policy."

US officials have said they do not expect the dispute over Abutalebi to hurt negotiations between Iran and six major powers under which Iran would restrict its nuclear program in exchange for economic sanctions relief.

Reuters