"We are not considering an alternative pick," deputy foreign minister Abbas Araqchi told Mehr news agency, referring to Hamid Aboutalebi, the Islamic republic's choice for ambassador to the United Nations.
The United States has said Aboutalebi would not be granted a permit to take up his post in New York because of his links to the student demonstrators who stormed the American embassy in Tehran in 1979.
In his comments today, Araqchi said the foreign ministry was pursuing the issue of visa denial "through legal mechanisms at the UN".
The row, stoked by fury in the US Congress, also presents a stiff challenge to President Barack Obama's push for a diplomatic breakthrough with Iran after decades of mistrust.
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As the host government of the United Nations, the US is obliged to issue visas to diplomats who serve at the New York-based institution.
It is believed that Washington has never before denied a visa for a UN ambassador, although Tehran withdrew its nominee once in the early 1990s.