TOKYO -- Japan's new government, already bickering with the United States about the location of a Marine air station on Okinawa, appears intent on revealing evidence of a decades-old secret pact between Tokyo and Washington that allowed U.S. ships and aircraft to carry nuclear weapons on stopovers in Japan.

Foreign Minister Katsuya Okada said the investigation into the pact is in its final stages, and that its findings will be announced in January. "We'll be unburdening ourselves of the insistence of past governments that a secret agreement did not exist," Mr. Okada said in a speech last weekend.

The pact violates a Japanese law that prohibits nuclear weapons from being made, possessed or stored on its territory. But disclosure of the 1960s-era agreement is hardly new. In general outline, its existence has been known for years because of declassified U.S. government documents.

Still, the government's insistence on an official investigation of the matter has placed a new strain on U.S.-Japanese relations. "This is not the type of issue your closest ally forces you to confront publicly," said Ralph Cossa, president of the Center for Strategic and International Studies' Pacific Forum, a think tank in Honolulu.

When Defense Secretary Robert Gates visited Japan last month, he reportedly told Japan's defense minister not to allow the investigation of the agreement to hurt bilateral relations or weaken U.S. nuclear deterrence. The U.S. government is treaty-bound to defend Japan in case of attack, and it has about 36,000 military personnel based here.

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