TOKYO, Nov. 20, 2009 (Kyodo News International) -- Foreign Minister Katsuya Okada has decided to admit the existence of a secret Japan-U.S. pact under which Tokyo allows stopovers of U.S. military vessels or aircraft carrying nuclear weapons, Foreign Ministry sources said Saturday.

During an in-house probe by the Foreign Ministry, documents suggesting the existence of the secret nuclear agreement were found, and Okada will announce the outcome of the investigation early next year, according to the sources.

Under the 1960 bilateral security treaty, Washington is required to consult with Tokyo before bringing nuclear weapons into Japan.

The ministry will determine that the documents indicate the stopovers of the U.S. military vessels or aircraft with nuke weapons are not subject to prior consultation, the sources said.

After assuming the ministerial post in September, Okada ordered his ministry to look into purported secret pacts between the two countries -- two related to the revision of the security treaty and two related to the 1972 reversion of Okinawa to Japanese sovereignty from U.S. control.

The Japanese government has consistently denied the existence of the secret nuclear pact, saying, ''As we have never faced demands for prior consultations, we have no other choice than determining that nuclear (weapons) have not been brought into Japan.''