The United States said it had told Iran the selection of a UN ambassador who is allegedly linked to the 1979 American embassy hostage crisis was "not viable".
The disclosure from the White House came a day after the US Senate passed a resolution that would deny the diplomat, Hamid Aboutalebi, a US visa.
"The US government has informed the government of Iran that this potential selection is not viable," White House spokesman Jay Carney said on Tuesday.
The clash over the ambassadorial 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 programme.
Iran has defended its appointment of Aboutalebi, brushing aside US concerns.
It says he is a veteran diplomat with a successful record and is as such qualified to serve at the UN.
Seen as close to the reformists and an ally of President Hassan Rouhani, Aboutalebi is currently the director general of the presidency's political affairs bureau.
He has previously served as ambassador to the European Union, Belgium, Italy and Australia.
The State Department has previously warned that Aboutalebi's nomination would be "extremely troubling".
Aboutalebi has insisted he was not part of the hostage-taking in November 1979, when students who had overthrown the pro-Western shah seized the US embassy, but that he later joined the student group.
He has said that 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 and poisoning relations between Washington and Tehran for a generation.
As the host government, the United States generally is obliged to issue visas to diplomats who serve at the United Nations, although there have been exceptions.
The Senate unanimously passed a resolution that would deny Aboutalebi an entry visa.
The House of Representatives is also expected to adopt the bill.