The diplomats, who spoke to The Associated Press on condition of anonymity, cautioned that the investigation was not yet complete and other sources still could not be ruled out.
Still, they said evidence increasingly points to the secretive communist country, leading to fears that it could have supplied other nations with fuel, components and knowledge needed to build nuclear weapons.
The evidence also focuses on the North's secret weapons program using uranium technology. North Korea, initially thought to have only a plutonium-based program, acknowledged developing a parallel program based on uranium enrichment after U.S. disclosures of its existence two years ago, but details remain sketchy.
Pakistan, the key country implicated in a worldwide black market nuclear network, had been thought to be the source of the 1.87 tons of uranium hexafluoride handed over to the Americans in January as part of Libya's voluntary decision to rid itself of weapons of mass destruction.
But "now, we believe that it might have been" North Korea who supplied the substance, said one of the diplomats. "It's a definite possibility."
The diplomat said the evidence from the Vienna-based International Atomic Energy Agency was based on interviews with members of the clandestine network headed by Abdul Qadeer Khan, the Pakistani scientist implicated in selling his country's nuclear secrets to Libya, North Korea, Iran, and possibly other countries.
In its raw form, uranium hexafluoride cannot be used as nuclear fuel — or in warheads. But it is part of the enrichment cycle, using centrifuges to separate isotopes in the process that can make nuclear fuel — or warheads. Libya had purchased hundreds of centrifuges as part of a multimillion dollar enrichment program, with the Khan network as the main supplier.
One of the diplomats said that despite its size, the shipment thought to have come from North Korea would only have been enough to make one small nuclear weapon. When it came clean on its weapons ambitions in December, Libya was far away from achieving that goal.
The evidence collected by the IAEA sheds more light on the North's program using uranium enrichment to make nuclear arms parallel to its better-known plutonium-based activities. North Korea is demanding U.S. economic aid and other concessions in exchange for scrapping its nuclear weapons programs.
Khan, the Pakistani nuclear scientist, has admitted that he provided North Korea with assistance for development of a uranium bomb, something the North Koreans deny
U.S. officials believe North Korea already has one or two nuclear bombs and could make several more within months.