The White House announced Friday that it will not grant a visa to Iran’s pick to be its United Nations ambassador, Hamid Aboutalebi, amid a controversy over his role in the 1979 hostage-taking at the American embassy in Tehran.

“We have informed the United Nations and Iran that we will not issue a visa to Mr. Aboutalebi,” press secretary Jay Carney told reporters at his daily briefing.

It was not clear whether Iran would challenge the decision at the United Nations or whether the unusual rejection would poison President Barack Obama’s diplomatic outreach to Tehran.

The announcement came after Congress overwhelmingly passed a bill designed to keep Aboutalebi out of the United States because of the part he played in the crisis, in which 52 Americans where held hostage 444 days. The diplomat has said that he took part in the standoff only after the seizure of the embassy and that his role was as a translator.

Carney would not say whether Obama would sign the legislation, citing concerns “related to its utility and its constitutionality.”

Carney and State Department spokeswoman Jen Psaki predicted that the refusal would not damage tense ongoing negotiations with Iran aimed at ensuring that country does not develop nuclear weapons.

Those talks are “moving forward in a workmanlike manner and ... we do not expect to be affected by this decision,” Carney said.

The spat over Aboutalebi “has been happening publicly while the negotiations were happening,” Psaki said. U.S. negotiators “don't feel there was an impact.”

Psaki refused to say precisely why the United States was refusing to let the Iranian — who has served previously as his country’s ambassador to Australia, Belgium, the European Union and Italy — onto U.S. soil.

“Details of visa cases, including the reasons … are not issues that we can talk about publicly for legal reasons,” she said.

According to the 1947 treaty that covers the U.N. headquarters in New York, the United States is supposed to grant visas for entry to individuals with legitimate business before the world body.

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“Laws and regulations in force in the United States regarding the entry of aliens shall not be applied in such manner as to interfere” with that access and “when visas are required for persons referred to in that Section, they shall be granted without charge and as promptly as possible,” the treaty says.

“We take our host country responsibilities very seriously, which is why this is such a rare case,” Carney said.

The State Department has said that it can withhold visas on grounds of “security, terrorism and foreign policy concerns” — all extremely broad categories -- but neither Psaki nor Carney would give a precise explanation.

“All of these issues are looked at by our legal teams, but I'm not going to give a specific reasoning,” Psaki said Friday.

It’s not the first time that the United States and Iran — which have not had formal diplomatic relations since the 1979 Islamic revolution that swept Ayatollah Ruhollah Khomeini to power — have battled over visas.

In 2005, Iran sought a visa for then-President Mahmoud Ahmadinejad. The State Department openly called into question his eligibility to visit the U.S. because of alleged ties to the same hostage crisis. It ended up granting the visa request just one week before the U.N. General Assembly in September, with the caveat that the fiery Iranian leader was forbidden to travel more than 25 miles from U.N. headquarters in New York.

And it’s not the first time that the United States has actually blocked a foreign official from getting to U.N. headquarters.

In 2013, the United States kept Sudanese President (and war crimes suspect) Omar al-Bashir out of New York by not acting on his visa request until after the U.N. General Assembly.

In 1988, the United States kept Palestine Liberation Organization leader Yasser Arafat out on grounds of “affiliation in an organization which engages in terrorism.”

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