Iran proposed a new envoy to the United Nations this week, but the U.S. has objected, citing the envoy's links to the 1979-1981 hostage crisis. (Reuters)

The Obama administration said Friday that it would block Iran’s nominee as ambassador to the United Nations from entering the United States, setting up a new confrontation with Tehran just as relations with the Islamic republic appeared to be improving.

The decision to bar entry to the diplomat, who was allegedly involved in the 1979 seizure of the U.S. Embassy in Tehran, followed intense political pressure on the administration from Republicans and Democrats on Capitol Hill. But it also marked a rare instance in which Washington has effectively vetoed another country’s preferred choice as envoy to the United Nations.

Iran quickly condemned the move.

“It is a regrettable decision by the U.S. administration, which is in contravention of international law, the obligation of the host country and the inherent right of sovereign member states to designate their representatives to the United Nations,” said Hamid Babaei, spokesman for Iran’s U.N. mission in New York.

White House press secretary Jay Carney did not offer a reason for the decision not to grant an entry visa to the diplomat, Hamid Aboutalebi, but said officials had “communicated with the Iranians at a number of levels and made clear our position on this.”

Negotiators from Tehran and six world powers struggle to narrow ‘significant gaps’ in a long-term nuclear deal. (Reuters)

Despite 35 years of diplomatic estrangement from the United States, Iran maintains a large and active mission to the United Nations. Iranian diplomats are confined to New York, but in keeping with long-standing practice as the U.N. host country, the United States routinely approves diplomatic passage for Iranian diplomats and leaders.

Aboutalebi has said that he was not part of the embassy takeover — in which 52 Americans were held hostage for 444 days — and that he only provided translation services later. The government of Iranian President Hassan Rouhani has said it stands by its nominee, without discussing any involvement in the events in 1979.

The administration’s decision is likely to be viewed by some other governments as a breach of the United States’ responsibilities as the host country for the United Nations.

A senior European diplomat said it is too soon to know whether there will be any organized opposition to the move but added that the United States’ reasoning for barring Aboutalebi will be carefully scrutinized. European nations are host to large numbers of international organizations, including many affiliated with the United Nations.

The diplomat requested anonymity because the details of Aboutalebi’s case are still murky.

There have been previous instances in which the United States has opposed entry to diplomats or heads of state seeking to travel to the United Nations, most often in cases in which they have been accused of terrorism or other crimes.

In the case of an Iranian nominee to the United Nations in the early 1990s and, more recently, in that of Sudanese President Omar Hassan al-Bashir, the requests for U.S. visas were withdrawn after the United States signaled opposition, or the State Department declined to process the application.

It was not clear whether the United States denied Aboutalebi’s visa request or merely shelved it. State Department spokeswoman Jen Psaki said the administration is barred from providing that level of detail because visa cases are confidential.

The first response from Tehran to the rejection of Aboutalebi’s application came from Alaeddin Boroujerdi, head of the Iranian parliament’s national security and foreign policy committee, who urged Iran’s foreign minister, Mohammad Javad Zarif, to write a letter of protest to U.N. Secretary General Ban Ki-moon.

“We chose Aboutalebi as our representative to the United Nations, and the United Nations headquarters are inside U.S. territory, so Americans, by rejecting his entrance, are taking advantage of the United Nations’ geographical location,” Boroujerdi said in a session of parliament Saturday morning.

The administration had hoped that Tehran would withdraw Aboutalebi’s name and thus avoid a full confrontation that could sour the mood for what both nations have called productive talks on Iran’s nuclear program. The next and potentially decisive round is due to begin May 14.

There was no immediate indication from Iran that it would withdraw the visa application. But on Wednesday, Zarif said blocking Aboutalebi from U.S. soil would be “unacceptable.”

Carney expressed confidence that the flap over Aboutalebi can be kept separate from U.S. dealings with Iran over its nuclear program.

“There’s a process in place,” Carney said, “one that is moving forward in a workmanlike manner and that we do not expect to be affected by this decision.”

International negotiators met last week in Vienna to further a deal that would roll back Iran’s disputed nuclear program in exchange for a suspension of the international economic sanctions that have hobbled the Iranian economy.

U.S. and Iranian envoys met separately for about an hour on the sidelines of the nuclear talks, U.S. officials said. That followed personal meetings between Secretary of State John F. Kerry and Iran’s foreign minister, and a surprise phone call by President Obama to Rouhani in September.

Rouhani was elected last year with a mandate to pursue nuclear talks and get out from under the yoke of sanctions. He has presented a friendlier face to the West than his predecessor, Mahmoud Ahmadinejad, with such innovations as an English-language Twitter feed.

Rouhani has hard-line opponents at home, however, and it is not clear whether Aboutalebi was his choice or theirs.

The White House decision comes one day after the House of Representatives voted unanimously to bar entry to the United States to those involved in terrorism or deemed a threat to U.S. security. The wording of the House legislation, like a Senate version approved previously, was designed to exclude Aboutalebi, even though his application for entry as a U.N. diplomat would ordinarily be approved.

The White House agrees with the intent of the legislation, Carney said, although he did not say whether Obama will sign it.

“This is simply the right thing to do,” Sen. Charles E. Schumer (D-N.Y.) said Friday. “Hamid Aboutalebi’s nomination would have been a slap at all American victims of terrorism, not just those taken hostage in 1979.”

Jason Rezaian in Tehran contributed to this report.