State media quoted a Foreign Ministry official as saying low-enriched uranium was being produced for the reactor.
North Korea announced the existence of the reactor earlier this month.
The US and South Korea have repeatedly demanded Pyongyang gives up its nuclear programme, which they fear is aimed at building atomic weapons.
The latest announcement is likely to be seen as provocative in Seoul and Washington.
The KCNA news agency carried quotes from an unnamed Foreign Ministry official saying: “The construction of experimental LWR [light-water reactor] and the low-enriched uranium for the provision of raw materials are progressing apace.”
The official described the reactor as part of the isolated regime’s attempts to build a “self-supporting national economy” and said the country was making “leaping progress” in science and technology.
US Secretary of State Hillary Clinton is currently visiting South Korea, and analysts say the timing of the announcement may have been intended as a message to the US.
North Korea agreed in 2005 to halt its nuclear programme in return for aid, and it was involved in six-nation talks on denuclearisation for four years.
But the North Koreans walked out of the talks in April 2009, and one month later carried out a nuclear weapons test.
Pyongyang is believed to be close to having the technology for a nuclear weapon, and the tests appeared to be a bid to find a delivery system for a warhead.