The United States has recently warned Japan that North Korea may possess several nuclear warheads that can be loaded onto ballistic missiles, government sources said Friday.

Previously, U.S. Secretary of State Colin Powell had stated that North Korea may have "one or two nuclear weapons." But the issue of whether Pyongyang has the technology to produce warheads small enough to be carried aboard such missiles has remained murky.

A senior government official said that several U.S. government officials relayed this information to Tokyo, although he did not specify when. One of the U.S. officials told Tokyo he was sure that North Korea possesses nuclear warheads, according to the Japanese official.

Meanwhile, a senior aide to Prime Minister Junichiro Koizumi said it would be difficult for Japan or any other country to verify claims of this kind unless North Korea conducts nuclear weapons test.

But Chief Cabinet Secretary Yasuo Fukuda did not rule out the possibility that Pyongyang possesses nuclear warheads. "The Japanese government has a great interest in North Korea's ballistic missiles and nuclear (weapons), and is in contact with a lot of information on the issue," Fukuda told a news conference. "But there is nothing we can say for sure at this point in time."

Defense Agency chief Shigeru Ishiba said that, although Pyongyang may be attempting to miniaturize nuclear warheads, he believes it would be "extremely difficult" for North Korea to be successful.

Koizumi said that Japan has no intention of altering its policy toward the North, urging the nation to resume dialogue with the U.S. and other countries.

During April trilateral talks with the U.S. and China in Beijing, North Korea admitted it has several nuclear weapons and has completed the reprocessing of nuclear fuel rods.