Iran has accused the US of acting illegally by refusing a visa to the new Iranian envoy to the UN in New York.
Iran’s UN mission spokesman Hamid Babaei called the decision to bar Hamid Aboutalebi “regrettable”, but did not specify any action Tehran might take.
The US accuses Mr Aboutalebi of links to the group that seized the US embassy in 1979, an incident that soured ties between the two countries for decades.
Mr Aboutalebi says he only acted as a translator for the group.
The UN has not yet commented on Mr Aboutalebi’s situation.