The United States said Tuesday 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 for the United States 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 radical Iranian students.
The protracted standoff profoundly shocked the United States 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 has blown up amid a cautious thaw in relations as Tehran's new leadership seeks to negotiate a nuclear treaty with global powers.
On Monday, Iran urged United Nations Secretary General Ban Ki-moon to intervene directly in the row, and the UN committee which oversees such visa issues will meet on April 22 to discuss the matter.
As the host government, the United States is generally obliged to issue visas to diplomats who serve at the United Nations. Aboutalebi has previously attended sessions at the UN headquarters in New York.
But Washington says there are "limited exceptions" including whether the US has concerns about security or terrorism involving the nominee.
- 'A visa we cannot grant' -
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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."
US officials have privately said the visa application has not been denied outright, and are hoping that Tehran will defuse the crisis and pick another nominee.
But on Monday Iran's foreign ministry spokeswoman Marzieh Afkham reiterated the Islamic republic's position.
"The official mechanisms for lodging the complaint at the United Nations have been activated, and are under way," she said in remarks published on state broadcaster IRIB's website.
It is believed that the US has never previously denied a visa for a UN ambassador, although Tehran withdrew its nominee once in the early 1990s.
A US official, who asked not to be named, said Washington had decided to break its silence on why it objected to Aboutalebi's nomination, saying "I think we felt it was important to communicate what the concerns were about."
Aboutalebi, a veteran diplomat who currently heads President Hassan Rouhani's political affairs bureau, has insisted he was not part of the hostage-taking in November 1979, when a Muslim student group seized the US embassy after the overthrow of the pro-Western shah.
He has acknowledged he served a limited role as a translator for the students who took the Americans hostage.
But US lawmakers, who passed a bill last week barring him from US soil, branded Aboutalebi a "terrorist" and said he should not be allowed to walk around the streets of New York with diplomatic immunity.