WASHINGTON - The White House announced on Friday that the US will not issue a visa to Iran's choice for UN Ambassador, over concerns about his reported involvement in the 1979 US Embassy hostage crisis in Tehran.The decision, which is bound to escalate tensions between Tehran and Washington, comes after Congress earlier this week approved a bill that would bar Hamid Abutalebi from stepping on US soil.White House Press Secretary Jay Carney said that the White House is reviewing that legislation but announced that Abutalebi would be barred anyway."We have informed the United Nations and Iran that we will not issue a visa to Abutalebi," Carney said."We certainly share the intent of the bill passed by Congress as we have already told the UN and Iran that we will not issue a visa." State Department spokeswoman Jen Psaki said it was "not a viable nomination." Denying visas to UN Ambassadorial nominees or to foreign heads of state who want to attend United Nations events in the United States is very rare, if not unprecedented. American officials, though, have objected to the selection of Abutalebi because of his alleged participation in a Muslim student group that held 52 Americans hostage for 444 days in the 1979 incident. The concerns became a rare point of bipartisan agreement in Congress. The House unanimously approved the legislation on Thursday by voice vote, four days after a similar vote in the Senate. The clash over the nomination also threatens to complicate a key moment in the easing of relations between US and Iran as both sides strive to conclude a deal on the Islamic Republic’s nuclear programme.Iran had already defended Aboutalebi’s appointment, brushing aside US concerns, and did so again on Thursday.Speaking to Iranian media in Vienna, where the latest round of nuclear talks is underway, Foreign Minister Muhammad Javad Zarif said “we announced to the Americans one of our most rational and experienced diplomats as our United Nations envoy.”“The government of the United States is well aware that this kind of behaviour is by no means acceptable for us,” he said, himself a former UN envoy.Seen as close to the reformists and an ally of President Hassan Rouhani, Aboutalebi is currently the Director General of Political Affairs Bureau of the President’s office. He has previously served as Ambassador to the European Union, Belgium, Italy and Australia.Aboutalebi has insisted he was not part of the hostage-taking in November 1979, when students seized the US Embassy after the overthrow of the pro-Western Shah.He has said 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. As the host government, the United States generally is obliged to issue visas to diplomats who serve at the United Nations, although State Department deputy spokeswoman Marie Harf has said there were “certain circumstances” for exceptions.