The CIA has admitted to orchestrating the August 1953 coup that toppled Iran's prime minister after he tried to nationalise his country's oil wealth from Britain, according to a declassified document.

George Washington University's National Security Archive, which obtained the documents under the Freedom of Information Act, a law that promotes government transparency, said that a secret internal history marked

the most explicit CIA admission, on Sunday.

"The military coup that overthrew Mosadeq and his National Front cabinet was carried out under CIA direction as an act of US foreign policy," the document said, using an alternative spelling of Mossadegh.

The CIA's role in the overthrow of Mohammad Mossadegh has long been known, with the coup haunting relations between the United States and Iran six decades later.

Mossadegh had angered Britain by moving to take over the Anglo-Iranian Oil Company, the predecessor of modern-day BP. The British believed that control of Iranian oil was vital to reviving their economy from the destruction of World War II.

The internal CIA history, released by the National Security Archive to mark the coup's 60th anniversary, offered a degree of understanding of Mossadegh's position and rejected Western media depictions of him as "a

madman" or "an emotional bundle of senility".

While recognising that the UK needed the oil, the CIA history said that British policymakers had "little in their experience to make them respect Iranians, whom company managers and Foreign Office managers saw as inefficient, corrupt and self-serving".

The CIA history also cast the decision in Cold War terms, fearing that the Soviets would invade and take over Iran if the crisis escalated and Britain sent in warships, as it would do three years later alongside France and

Israel when Egypt nationalised the Suez Canal.

"Then not only would Iran's oil have been irretrievably lost to the West, but the defense chain around the Soviet Union which was part of US foreign policy would have been breached," it said.

The coup strengthened the rule of the shah, Mohammad Reza Pahlavi, who became a close US ally. He was toppled in the 1979 Islamic revolution, with the new leadership making hostility to the United States a cornerstone of Iran's foreign policy.

Then Secretary of State Madeleine Albright, in an effort to mend relations with Iran, admitted in 2000 that the United States "played a significant role" in overthrowing Mossadegh and called the coup "a setback for Iran's political development".

President Barack Obama made a similar admission after taking office in 2009 in another unsuccessful attempt at reconciliation with Iran, which has sought an explicit apology.

Another internal history, authored by coup planner Donald Wilber and leaked to The New York Times in 2000, said that agents arranged stories against Mossadegh in the press, both in Iran and the United States, aimed at setting the stage for the coup.

US and British spies also tried to fan the flames against Mossadegh among the Iranian clergy, it said.

The history said that the CIA also arranged to pay $5 million within days of the coup to the new government of Fazlollah Zahedi, a general appointed to succeed Mossadegh.

Malcolm Byrne, deputy director of the National Security Archive, said that the CIA wrote the secret histories for internal use.

The histories "give people on the inside a sense of what happened and, presumably, give them a little context for whatever else they may be planning," he said.