The United States may soon deny a visa to Iran's proposed UN ambassador, two US officials said on Wednesday, a rare and potentially precedent-setting step that would test US influence over the world body.The US government objects to Hamid Abutalebi entering the United States because of his suspected participation in a Muslim student group that held 52 Americans hostage for 444 days starting in 1979, when the group seized control of the US embassy in the Iranian capital Tehran.Iran on Wednesday rejected US reservations about the veteran diplomat as "unacceptable." Abutalebi has played down his role in the embassy takeover, saying that he was only a translator for some of the militants. UN officials, diplomats and academics could not recall past cases of the United States denying a UN ambassador's visa. Some expressed concern over the precedent it could set."There would be a strong feeling internationally, or at least among diplomats, that that is not within the purview of the host country," said Jeffrey Laurenti, a veteran UN analyst and former fellow at the Century Foundation, a New York public policy research group.Two US officials familiar with the matter said the ban could be announced soon and was being debated by top Obama administration policymakers. The officials asked not to be identified because they are not authorized to comment publicly.A third US official said Iran may have chosen Abutalebi as an "intentional provocation" given sensitivities over the hostage crisis, a defining moment in US-Iranian relations that led the United States to cut diplomatic relations with Iran.The crisis also became a morale-sapping drag on Jimmy Carter's presidency, and is widely viewed as a significant factor in his loss to Ronald Reagan in the 1980 presidential election. On Tuesday, the Obama administration told Tehran that Abutalebi's nomination was "not viable." That drew a strong rebuke from Iran on Wednesday."The attitude of the US government towards Iran's (choice) for UN envoy is not acceptable. Iran has officially conveyed its views," IRNA, Iran's state news agency, quoted a Foreign Ministry spokesperson as saying.Abutalebi was among Iran's best and most experienced diplomats, the spokesperson said, noting that he had "ambassadorship experience" in Italy, Belgium and Australia.So far at least, the controversy does not appear to have influenced sensitive negotiations between major powers and Iran over curbing Iran's nuclear program in exchange for easing of economic sanctions, diplomats say.A DIFFICULT LEGAL QUESTIONAs the host nation for the UN headquarters, the United States generally is required to provide foreign diplomats access to the United Nations.However, the State Department said last week that US law allows it to deny diplomats visas for reasons of "security, terrorism, and foreign policy" - categories that a department spokeswoman acknowledged could be interpreted liberally. The right to deny entry to Abutalebi poses a knotty legal question that pits the US government's authority to police its own borders against the agreement the United States has with the United Nations, in which it pledges to allow envoys transit from the nation's border to the headquarters building in New York.Further muddying the waters is a 1947 Joint Resolution of Congress, which said nothing should be seen as "diminishing, abridging, or weakening the right of the United States to safeguard its own security and completely control the entrance of aliens" into any part of the United States aside from the U.N. headquarters."The State Department has relied on this reservation to deny visas to individuals deemed to pose a security risk to the United States," said John Bellinger, the top legal adviser to the State Department from 2005 to 2009.In an interview, Bellinger said the issue remains in dispute because some countries question whether the United States has such rights regarding diplomats.Last year, Sudanese President Omar Hassan al-Bashir was unable to travel to New York for the UN General Assembly in September after his visa application was left pending, according to Sudanese officials. Bashir has been indicted for war crimes in Sudan's Darfur region by the International Criminal Court.In 2012, the United States denied visas to about 20 Iranian government officials hoping to attend the General Assembly, including two ministers, Iran's Fars news agency reported.If it chose, Iran could complain directly to UN Secretary-General Ban Ki-moon about Abutalebi's case.The 1947 host country agreement also says disputes over the agreement should be referred to a tribunal of three arbitrators: one each chosen by the United Nations and United States and the third agreed to by both or appointed by the president of the International Court of Justice.