Chosun Ilbo newspaper quoted an unnamed government source as saying the North had started building a fence and reinforcing troops around the site of the 2006 underground nuclear test at Punggyeri in the northeastern county of Kilju.

"US spy satellites have spotted barbed-wire fences being built and troops being reinforced at the nuclear site in Punggyeri," the source told Chosun, the largest circulation daily in South Korea.

South Korean and US intelligence authorities, closely monitoring the activities, suspect the increase in security "aims to block outsiders from taking soil samples" from the nuclear test site, Chosun said.

The National Intelligence Service, Seoul's main spy agency, refused to confirm the report immediately, while other South Korean officials also would not comment on the intelligence matter.

North Korea shocked the world when it conducted its first nuclear test on October 9, 2006 in defiance of international warnings.

But the communist state soon returned to six-nation negotiations aimed at ending its nuclear weapons ambitions and agreed to an aid-for-disarmament deal.

North Korea has agreed to disable by December 31 the main reactor at Yongbyon and two other key nuclear facilities at the complex, which were shut down in July in the first phase of the disarmament accord.

North Korea is also required by the end of the year to make a full declaration of its nuclear programmes under the deal clinched by China, the United States, Russia, the two Koreas and Japan.

In return, the energy-starved communist state will get a total of one million tonnes of heavy fuel oil or equivalent economic aid plus major diplomatic and security benefits.

The United States has promised to work towards removing the North from its list of state sponsors of terrorism, and eventually normalising ties after full denuclearisation.