(AFP) -- US authorities have unofficially told their Japanese counterparts that North Korea already possesses several small nuclear warheads for ballistic missiles, a news report said today.

It is the first confirmation that Pyongyang has nuclear missiles that can immediately strike Japan, the Sankei Shimbun said, citing sources related to Japan and the United States.

US Defence Secretary Donald Rumsfeld and other US officials have publicly confirmed that North Korea was developing nuclear weapons.

Until now, however, they have not been clear about what stage of development North Korea was at, the Sankei said.

Washington has told Tokyo that the number of nuclear warheads that North Korea has is "not just one or two," the Sankei said.

It was not clear how the United States obtained the information, nor whether those weapons were developed or purchased by North Korea, the Sankei said.

Washington disclosed the information to Tokyo around March, the Sankei said.