TEHRAN, Iran - The United States notified Iran's government on Tuesday that Tehran's pick for a new U.N. envoy is "not viable," a day after the U.S. Senate voted against letting the diplomat enter the country.

American officials objected to the selection of Hamid Abutalebi because of his alleged participation in a Muslim student group that held 52 Americans hostage for 444 days in the 1979 seizure of the U.S. Embassy in Tehran. The Obama administration has stopped short of saying it would refuse him a visa.

"The U.S. government has informed the government of Iran that this potential selection is not viable," White House spokesman Jay Carney said in Washington.

Earlier, Iran's foreign ministry said it was waiting for a formal response from Washington.

The U.S. Senate on Monday approved a bill that would bar Abutalebi from entering the United States. Prospects for the bill in the House are unclear, though it is expected to have strong support.

Carney said the Obama administration finds Iran's choice of Abutalebi extremely troubling and shares the Senate's concern.

Carney did not say whether the U.S. would deny Abutalebi entrance into the country. He noted that U.S. responsibilities as the host nation for the U.N. could come into play.

Abutalebi, who previously served as Iran's ambassador to Belgium, the European Union, Italy and Australia, reportedly has insisted that his involvement in the group Muslim Students Following the Imam's Line was limited to translation and negotiation.

Calls to officials in Tehran were not immediately returned on Tuesday to find out if Abutalebi was a member of the student group.

However, a former member of the group, Bijan Adibi who had close contacts with John Limbert, one of the American hostages during the crisis, told The Associated Press that Abutalebi was not known as a member. "As far as I remember, Abutalebi was not in the main circles of those who occupied the embassy then. I think some in Washington are just using this as a pretext to make a barrier between Tehran and Washington."

Some Iranians who were closely linked to the U.S. Embassy seizure later moderated their views toward the U.S. and the West. Several have held official posts in various administrations since the Islamic Revolution.

Masoumeh Ebtekar, once a spokeswoman for the student group, is now vice president in charge of environmental affairs in President Hassan Rouhani's administration.

Foreign-based Farsi websites claim that the minister of economy and financial affairs, Ali Tayebnia, was reportedly another member of the student group. He visited Washington to attend International Monetary Fund and World Bank meetings in October.