US outrage at Iran's pick of 'hostage-taker' envoy
The Obama administration says Iran's nomination of a former hostage-taker as its ambassador to the United Nations is "extremely troubling".
US senators have also balked at Iran's pick of Hamid Aboutalebi, who was part of a Muslim student group which seized the US embassy in Tehran in 1979.
The 52 Americans were held for 444 days during the crisis.
Senator Ted Cruz says he will introduce legislation to block Iran's application for a US visa for Mr Aboutalebi.
'In your face'
Department of State spokeswoman Marie Harf said at Wednesday's daily briefing: "I will say that we think this nomination would be extremely troubling.
"We're taking a close look at the case now, and we've raised our serious concerns about this possible nomination with the government of Iran."
Officials for Iran's Mission to the United Nations have so far declined to comment.
Sources familiar with Mr Aboutalebi tell the BBC that he is one of the closest diplomats to Iranian President Hassan Rouhani.
Mr Cruz, a Texas Republican, said on the Senate floor on Tuesday: "It is unconscionable that in the name of international diplomatic protocol, the United States would be forced to host a foreign national who showed a brutal disregard of the status of diplomats when they were stationed in his country."
"This person is an acknowledged terrorist," he added.
His legislation would require US President Barack Obama to deny a visa to any UN applicant determined to have engaged in terrorist activity.
Fellow Republican Senator John McCain called Mr Aboutalebi's appointment "a really kind of an in-your-face action by the Iranian government", the Associated Press news agency reports. Democratic Senator Chuck Schumer has called him a "major conspirator" of the hostage crisis.
In an interview with an Iranian news site, Mr Aboutalebi said he was not part of the group that took over US embassy and was only later asked to translate for the students.
BBC Persian's Bahman Kalbasi says Ebrahm Asghar-Zadeh, a known member of the core group of hostage takers, corroborated Mr Aboutalebi's account in an interview.
"Occupying the embassy was the idea of five people, then expanded to 15," Mr Asghar-Zadeh said.
"After the embassy takeover, we invited close to 150 to 200 students to help us with different tasks of running the place. Some of the students would only commit to a few hours.
"I don't remember Mr Aboutalebi from those days but if he was, must have been one of those guys who was asked to help translate for reporter or a delegation. Calling him a hostage taker is simply wrong."
The son of another member of the group of five students who took over the embassy also told the BBC: "The names of the first five and the 10 that joined to execute the takeover are public and there is no doubt that Mr Aboutalebi was not in any way a major player in planning or execution of the hostage taking of US embassy staff in Tehran."