For the first time, there are reports that North Korea may have sold or transferred nuclear fuel to another country. U.S. experts have long warned that Pyongyang might trade its nuclear materials, and the latest reports could raise tensions between Washington and North Korea.

The New York Times reports that U.S. intelligence officials believe North Korea transferred two tons of uranium hexafluoride to Libya sometime before Tripoli abandoned its efforts to build nuclear weapons.

Experts say the substance can be used to create enriched uranium - which can be used to make nuclear weapons.

Kim Tae-woo, an analyst with the Korean Institute of Defense Analyses in Seoul, says if the report is confirmed, it would show that North Korea has mastered at least part of the process of refining uranium ore into weapons-grade material.

"Uranium hexafluoride is very sensitive material, because it is a transformed form of uranium that is ready to be enriched," he said. "So it is quite different from uranium ore."

Mr. Kim says he cannot confirm The New York Times  report, but adds it is likely to be true, given the past attempts of both North Korea and Libya to build nuclear weapons.

Libya last year agreed to dismantle its weapons program and turned over its stockpile of nuclear materials to international inspectors. Recently, Libyan President Muammar Qaddafi urged North Korea to renounce its nuclear ambitions.

If the report is confirmed, it would be the first known time North Korea is shown to have transferred nuclear material outside the country.

U.S. officials have previously warned such a transfer could increase the possibility of military action against North Korea. Washington fears that North Korea would sell nuclear material to terrorists or other rogue nations that might use it in a weapon against the United States or its allies.

The United States says Pyongyang has a secret program to enrich uranium to make nuclear weapons. North Korea denies having such a program, but says it has reprocessed plutonium to make another type of nuclear bomb.

Both programs violate several agreements North Korea has signed to remain free of nuclear weapons.

This latest report may have serious implications for the six-party talks aimed at persuading Pyongyang to give up its nuclear ambitions. The United States, China, Russia, Japan, and North and South Korea have held three unsuccessful rounds of talks but a fourth round has yet to be scheduled.