Pyongyang made the offer during a visit by the New Mexico governor at a time of heightened tensions, after it launched a deadly artillery strike on a South Korean island and unveiled a new uranium enrichment facility last month.
Japan's Kyodo News agency, citing unnamed sources, reported that the communist regime had however attached "certain conditions" to the offer to readmit inspectors of the UN International Atomic Energy Agency (IAEA).
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One of these conditions related to its offer to negotiate the sale of 12,000 of its nuclear fuel rods -- which are capable of producing bomb-making plutonium -- to a third party, possibly South Korea, the report said.
North Korea is seeking payment "about five times higher than the market price" for the fuel rods, the sources were quoted as saying by Kyodo.
The sources suspect North Korea may first want to invite IAEA monitors to the uranium enrichment facility in its nuclear complex in Yongbyon, near the capital, so it can back its claim that the project is for civilian purposes.
The North, Richardson said after his trip, had also agreed to consider a military commission grouping the two Koreas and the United States to prevent conflicts in disputed sea areas, and to reconnect a crisis hotline.
Richardson said the United States should consider a resumption of six-nation talks -- with China, Japan, the two Koreas and Russia -- under which the North earlier agreed to give up its nuclear weapons in return for aid.
The United States, South Korea and Japan have been wary of the latest reported concessions from the regime, which has twice tested nuclear weapons, test-fired missiles, and last year walked out of the denuclearisation talks.
"North Korea talks a great game. They always do. The real issue is what will they do," US State Department spokesman Philip Crowley said at the time.
