The National Intelligence Service (NIS) said Pyongyang probably purchased materials in 2000 to produce the prototypes for centrifuges needed for an enriched uranium program that would be the first step towards building a nuclear arsenal.

But tight international surveillance had prevented the Stalinist state from obtaining other key equipment, the report said.

"We believe North Korea has not reached the stage of building a HEU (highly enriched uranium) plant because of enhanced efforts by the international community to stop it from securing key equipment," Yonhap news agency quoted the NIS report as saying.

Two weeks ago, Pyongyang said it had made atomic weapons as it pulled out of six-party nuclear talks.

Pyongyang has made similar assertions in the past, and analysts say its latest boasts conform with Washington's assessment that North Korea has developed one or two crude bombs.

In October 2002, the United States accused North Korea of operating a secret nuclear program, sparking a standoff between the two countries.

At the time, US officials said North Korea privately admitted running the program, only to issue public denials later.

Since North Korea expelled international monitors from its nuclear complex two years ago, there has been no way to verify such claims.

North Korea is also developing rocket engines for its Taepodong-2 missile with a range of 6,700 kilometers (4,150 miles), the intelligence agency said.

Pyongyang stunned the world in 1998 by test-launching over Japan a Taepodong-1 missile with a range of up to 2,500 kilometers, causing concern that the country could use its ballistic arsenal to deliver nuclear warheads.