The US says it will not issue a visa to Iran's chosen UN envoy Hamid Aboutalebi because he was involved in the 1979 hostage crisis at the US embassy in Tehran.
"Given his role in the events of 1979, which clearly matter profoundly to the American people, it would be unacceptable to grant this visa," State Department spokeswoman Jen Psaki told reporters.
It was the first time that Washington has publicly linked Aboutalebi to the hostage crisis when some 52 American diplomats and staff were held for 444 days by Iranian students.
The protracted standoff profoundly shocked the US and led to the severing of all diplomatic ties between the US and Iran for the past three decades.
The spat over Aboutalebi's nomination to be the new envoy to the UN has blown up amid a cautious thaw in relations as Tehran's new leadership seeks to negotiate a nuclear treaty with global powers.
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On Monday, Iran urged UN Secretary General Ban Ki-moon to intervene directly in the row.
As the host government, the US is normally obliged to issue visas to diplomats who serve at the UN. Aboutalebi has previously attended sessions at the UN headquarters in New York.
But there are "limited exceptions" including whether the US has concerns about security or terrorism involving the nominee.
Psaki said Washington was "not going to get into any specifics of what we do or don't think he was involved in during the hostage crisis" adding "he himself has said he was involved".
"As we all know, this was a searing experience for 52 American citizens who were held hostage. And for that reason, you know, this is a visa we cannot grant."
