Iran is threatening to challenge the U.S. decision to deny a visa for its newly appointed UN ambassador. 

The State Department called the appointment unacceptable given the Iranian official's role in the 1979 takeover of the U.S. embassy in Tehran. 

Radical Iranian students held 52 Americans hostage for more than a year during the crisis. 

Iran says it will not replace the diplomat and is vowing to take the dispute directly to the United Nations.