North Korea can make a nuclear warhead that can be mounted on missiles capable of hitting all of South Korea and most of Japan, U.S. nuclear experts said in a report obtained on Wednesday.

Two U.S. nuclear experts who recently visited North Korea's atomic facilities north of Pyongyang said the secretive and impoverished state had separated enough plutonium for five to 12 nuclear weapons.

"Little is known about North Korea's ability to make a nuclear weapon, although it is assessed as likely able to build a crude nuclear warhead for its Nodong (Rodong) missile," the Washington-based Institute for Science and International Security said in a report released on Tuesday in the United States.

The non-governmental group's report added: "the warhead may not be reliable, and it may have a relatively low yield."

Many proliferation experts doubt whether North Korea has the ability to miniaturize a nuclear warhead to fit on a missile.

The ISIS report said North Korea had probably obtained technology from overseas that would help it make a crude nuclear warhead.

Experts do not doubt that North Korea has hundreds of missiles, including its modified Scud-type missile called the Rodong, that are capable of hitting all of South Korea and large parts of Japan.

North Korea conducted its first nuclear test in October. Seoul government officials and nuclear experts said the exploded device had a relatively low yield.

North Korea agreed earlier this month to shut its sole reactor and main source of plutonium in return for aid as a step toward scrapping its nuclear arms program.