Librarian of Congress James Billington is leading a small delegation on the rare trip, which also comes as Iranians celebrate the 25th anniversary of the November 1979 seizure of the US embassy in Tehran during the country's Islamic revolution, the officials said.

"The purpose of Dr Billington's trip to Iran is purely cultural," said Helen Dalrymple, a spokeswoman for the Library of Congress. "The Library of Congress is interested in expanding its collection of Iranian publications.

"The process of collection has been curtailed since the Islamic Revolution in 1978-79, and the library wishes to ensure that Congress is well served with printed, digital and other materials in different formats that are available not only in Persian but also in the other languages of Iran," Dalrymple said.

She said Billington had been invited by his counterpart Muhammad Kazem Mousavi Bojnourdi, director of the National Library of Iran.

State Department spokesman Richard Boucher said Billington's trip had been approved by the White House and that he and his delegation had been briefed by the State Department before they left.

But he downplayed any suggestion that the trip was intended as a symbolic overture to Iran or as a mission that might ease US concerns about Iran's nuclear program, its opposition to the Israeli-Palestinian peace process or its support for alleged terrorist organizations.

"I don't attach a particular importance to this exchange," he told reporters. "I don't think the fact that the Librarian of Congress is going to Iran is going to change Iranian attitudes or behavior ... Topic after topic, we have serious problems with the Iranian government."

The Federation of American Scientists, which first reported Billington's trip, said the visit had been arranged by a private organization known as Catalytic Diplomacy and was intended to explore and seal a deal on an exchange program.

The federation said the visit was to end on Friday after the conclusion of an exchange agreement between the Library of Congress and Iran's national library.

The library, the research arm of Congress and the national library of the United States, is the world's largest, and its chief is appointed by the president and confirmed by the Senate.

The director of Catalytic Diplomacy, Jeremy Stone, organized a scientific exchange agreement between the non-governmental National Academy of Sciences and the Iranian Academy of Sciences in 1999, but formal contact between Washington and Tehran at Billington's level is unusual.

The United States and Iran severed diplomatic relations after the seizure of the embassy, which resulted in the taking of 52 American hostages who were held captive for 444 days by Islamic students.

In Tehran on Wednesday, thousands of Iranians gathered at the former US embassy, chanting "Death to America" and carrying posters depicting the Statue of Liberty with a death mask.

As in past years, demonstrators burned the Stars and Stripes along with the flag of Israel, the closest US ally in the Middle East, and as the rally broke up, organizers read a statement saying that Iran would refuse to give up its right to nuclear technology.

The United States accuses Iran of secretly trying to develop nuclear weapons under cover of a civilian atomic energy program, and the International Atomic Energy Agency (IAEA) was to refer the matter to the UN Security Council later this month for possible sanctions.

Iran has vehemently denied the US allegations and is fighting Washington's efforts to send the matter to the Security Council.

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