TEHRAN — Iran described as “unacceptable” Wednesday a decision by the United States to deny a visa to Tehran’s newly appointed UN ambassador over his alleged links to the 1979 US hostage crisis.

The clash over the nomination threatens to complicate a key moment in the easing of relations between Washington and Tehran as both sides strive to conclude a deal on the Islamic republic’s nuclear program.

On Tuesday, a day after the US Senate passed a resolution that would deny a visa to Hamid Abutalebi, White House spokesman Jay Carney said his selection was “not viable.”

Iran had already defended Abutalebi’s appointment, brushing aside US concerns, and did so again on Wednesday.

Speaking to Iranian media in Vienna, where the latest round of nuclear talks is underway, Foreign Minister Mohammad Javad Zarif said “we announced to the Americans one of our most rational and experienced diplomats as our United Nations envoy.

“The government of the United States is well aware that this kind of behavior is by no means acceptable for us,” added Zarif, himself a former UN envoy.

Seen as close to the reformists and an ally of President Hassan Rouhani, Abutalebi is currently the director general of political affairs bureau of the president’s office.

He has previously served as ambassador to the European Union, Belgium, Italy and Australia.

Abutalebi has insisted he was not part of the hostage-taking in November 1979, when students seized the US embassy after the overthrow of the pro-Western shah.

He has said he worked as a translator when the students, soon after the hostage-taking, released 13 women and African Americans to highlight what they said was Islamic respect for women and poor US treatment of minorities.

The remaining 52 diplomats spent a total of 444 days in captivity, enraging the United States.

As the host government, the United States generally is obliged to issue visas to diplomats who serve at UN headquarters.

But State Department spokeswoman Jen Psaki said Wednesday that there were “limited exceptions.”

They included “security, terrorism and foreign policy concerns. So there are a broad range of broadly speaking reasons that a visa could be deemed ineligible,” Psaki told reporters.

She refused to discuss the specifics of Abutalebi’s case however, and whether he had submitted a visa request or not, citing privacy concerns.

Senator Ted Cruz, a Republican from Texas and vociferous critic of Iran, introduced the Senate resolution backed by fellow lawmakers that blocks US visas for “known terrorists” to represent their countries at the United Nations.

In his remarks Wednesday, Zarif referred to what he called a “group of extremists (that) must not be allowed to determine the agenda of Iran’s presence in the United Nations.”

The US and Iran still do not have diplomatic relations, but Rouhani and US President Barack Obama have taken steps to ease tensions through a tentative agreement to freeze parts of Iran’s nuclear program.