U.S. intelligence agencies relied on such tactics as spies and spy planes planes to closely monitor France's nuclear weapons capability following World War II, newly declassified documents show.

The documents, produced by the CIA and several other U.S. agencies, were posted on the Internet by the Jeffrey T. Richelson, a senior researcher at the National Security Archive, a freedom-of-information activist group.

Richelson said while America's interest in other nations' nuclear ambitions was always known, the new documents disclose some of the specific techniques used to gather intelligence on its allies.

"The United States was simply concerned with any other country that might get into the nuclear weapons field, whether friend or foe," Richelson told The Associated Press. "The intelligence requirement was to know what they were up to."

The documents show that French nuclear activities were of sufficient concern to prompt the Manhattan Project's intelligence section to produce a series of reports in 1946 on the possibility that French scientists were willing to sell nuclear know-how to the right bidder and described the French search for uranium deposits.

Besides spies and spy planes, the documents showed that the U.S. also used informers, ships, satellites and National Security Agency communications intercepts to track French atomic tests.

The recently released papers were also produced by interagency groups, the Manhattan Project's Foreign Intelligence Section, the State Department, the U.S. Pacific Command and the Strategic Air Command

The reports were based on a variety of sources on French nuclear research and development activities.

The revealed that in the late 1950s, France established an organization to develop a bomb and began its search for a test site in the Sahara Desert, culminating in a first atomic test in February 1960.

Monitoring the French nuclear test site at Mururoa Atoll in the South Pacific led to the only aircraft-carrier-based launches of U-2 spy planes ever attempted, two sorties in May 1966 off the USS Ranger, said Richelson, who obtained the documents while researching his recently published book, "Spying on the Bomb: American Nuclear Intelligence from Nazi Germany to Iran and North Korea."

The most recent document distributed Tuesday is a declassified 1987 State Department summary of France's theory of nuclear deterrence against the threat of a Soviet invasion. In all, Richelson has posted more than 30 of the documents.

The latest release reflects America's interest in preserving its own atomic secrets and learning what others were up to — an effort that began in 1943 with attempts to track Germany's evolving nuclear program, Richelson said.

France was the fourth nation to obtain nuclear weapons after the United States, the Soviet Union and Britain.

Richelson's National Security Archive is affiliated with George Washington University, in Washington, D.C.