The United States said on Friday it would not grant a visa to Iran's proposed United Nations ambassador, citing the envoy's links to the 1979-1981 hostage crisis, in a rare step that raises questions about how much influence the White House can wield over the world body.
President Barack Obama had come under strong pressure not to allow Hamid Abutalebi into the country to take up his position in New York, raising concerns that the dispute would disrupt delicate negotiation between Tehran, Washington and other world powers over Iran's nuclear program.
White House spokesman Jay Carney said the United Nations and Iran had been told "that we will not issue a visa to Mr Abutalebi." Neither the White House nor the State Department provided further explanation.
US law allows the government to bar UN diplomats who are considered national security threats, but Obama's potentially precedent-setting step could open the United States to criticism that it is using its position as host nation to improperly exert political influence.
The US government objects to Abutalebi because of his suspected participation in a Muslim student group that seized the embassy in November 1979 and held 52 Americans hostage for 444 days.
The veteran diplomat has acknowledged that he acted as an interpreter for the militants who held the hostages.
Obama's decision came days after negotiators from Iran, the United States and five other world powers met in Vienna for another round of nuclear talks.
A spokesman for Iran's mission to the United Nations said the White House decision was unfortunate and may violate international law.
"It is a regrettable decision by the US Administration which is in contravention of international law, the obligation of the host country and the inherent right of sovereign member states to designate their representatives to the United Nations," spokesman Hamid Babaei said in a statement.
But an Iranian official said he did not expect the dispute to affect the nuclear negotiations.
Any official response would be up to the Iranian Foreign Ministry, but the US decision "will have no impact on our talks with the P5+1," the official told Reuters.
US officials also said they did not expect any impact.
The United Nations said it had no comment at this time on the US decision.
Obama's decision raised concerns about possible precedents, such as calls from US interest groups for future visas to be denied for political reasons, or retaliation abroad.
"If the US starts to pick and choose who can represent other countries at the UN, other countries are likely to react angrily. How would Washington feel if Switzerland vetoed its choice for American ambassador to the Human Rights Council in Geneva?" said Richard Gowan, an international relations expert at New York University.
After former hostages objected to Abutalebi, members of Congress jumped to pass legislation banning him, seeing the issue as a chance to look tough on Iran after a new sanctions bill stalled in the Senate early this year.
Unusually, the legislation passed unanimously in both chambers of the normally divided Congress this week.
Many members of Congress, even Obama's fellow Democrats, are deeply skeptical about Iran, despite the administration's efforts to ease tensions with the long-time US adversary. They had made clear they considered Iran's selection of Abutalebi as a rebuke of the United States.
"I am grateful that President Obama made the right decision and will deny the visa for this Iranian terrorist," US Representative Doug Lamborn, a Colorado Republican who sponsored the bill in the House of Representatives, said in a statement.
The White House is still reviewing the legislation, which would bar any UN representative deemed to be behind acts of terrorism or espionage against the United States. It would need Obama's signature to become law.