Iran has rejected U.S. reservations over Tehran's proposed UN ambassador.
Washington objects to Hamid Abutalebi entering the United States because of his suspected role in the 1979 U.S. Embassy takeover in Tehran and the hostage-taking of 52 Americans there for 444 days.
On April 8, the White House said the United States had told Iran that Abutalebi's nomination was "not viable."
That drew a strong rebuke from Iran the next day.
On April 9, an Iranian Foreign Ministry spokesman rejected such U.S. reservations as "unacceptable."
Quoted by Iran's state news agency IRNA, Marzieh Afkham said Abutalebi was among the country's top diplomats, with "ambassadorship experience" in Italy, Belgium, and Australia.
Abutalebi denies playing any role in the embassy seizure, saying he only served as a translator to the hostage-takers.
Analysts says Abutalebi is close to Iranian reformists as well as being an ally of President Hassan Rohani, who has pledged to improve ties with the West.
Abutalebi is currently the director-general of the political-affairs bureau of the president's office.
Analysts say as the host nation for the UN headquarters, the United States generally is required to provide foreign diplomats access to the United Nations.
However, the State Department has said that U.S. law allows it to deny diplomats visas on grounds of "security, terrorism, and foreign policy."
According to Iran's Fars news agency, in 2012, the United States denied visas to about 20 Iranian government officials hoping to attend the UN General Assembly.
U.S. lawmakers are already taking steps to deny Abutalebi entry to the United States.
On April 7, the Senate unanimously passed legislation to that end.
Senator Ted Cruz (Republican-Texas), the sponsor of the bill, said the "nomination is a deliberate and unambiguous insult to the United States."
Representative Doug Lamborn (Republican-Colorado) has introduced similar legislation in the House of Representatives.
With reporting by AFP and AP