But Pyongyang stopped short of admitting that it had begun the process of uranium enrichment, Japan's Kyodo News said quoting unnamed diplomatic sources.

The revelation was made by Kim Kye-Gwan, North Korea's chief delegate to multilateral talks on its nuclear programmes, in a meeting with his US counterpart Christopher Hill in Geneva earlier this month, the report said.

The report did not name the third country that allegedly supplied the aluminum pipes to North Korea.

In a landmark six-nation deal brokered in February, North Korea agreed to dismantle all of its nuclear facilities and programmes in exchange for diplomatic concessions and energy and other aid.

The parties to the six-nation talks, which began in 2003, are the two Koreas, China, Japan, Russia and the United States.

The United States has said it believes North Korea is running a secret uranium enrichment programme in addition to the programmes it has declared. Pyongyang has refuted that allegation.