A US scientist who travelled to North Korea this week said Saturday he had been told by officials there that the North had started building a light-water nuclear reactor.

"They were saying that they are constructing a small experimental light-water reactor (in Yongbyon), eventually of the size of about 25 to 30 megawatts," Siegfried Hecker said in footage by the Japanese public broadcaster NHK.

Hecker, a regular private visitor to North Korea who was formerly director of the United States' Los Alamos National Laboratory, had been in North Korea since Tuesday, the broadcaster said. He made the commments while passing through Beijing.

Light-water reactors are generally used for civilian nuclear purposes. Experts say it is difficult to use them for extracting plutonium to make weapons.

In 1994 the United States reached an agreement with long-time foe North Korea by which several countries were to build two light-water reactors for the North. In return, the North was to shut down its plutonium producing reactor in Yongbyon.

However the deal broke down in 2002, the light-water reactors envisaged at the time were never built, and the North restarted its plutonium-producing reactor.

The start of construction of a light-water reactor would therefore suggest progress in the country's own nuclear energy programme.