North Korea appears to be preparing for a third nuclear test, a South Korean newspaper said on Thursday, citing a government source.
US satellites detected movements of personnel and vehicles at the location where the reclusive communist state carried out its first two nuclear tests, the Chosun Ilbo said.
But a South Korean defence ministry official told AFP on condition of anonymity that such movements were being constantly detected, possibly for the daily maintenance of key strategic facilities there.
"Hectic movements of personnel and vehicles have recently been detected in Punggye-ri," Chosun quoted the unidentified government source as saying.
The North also appears to be restoring tunnels demolished during the first two tests, according to the source.
"However, it is unlikely (the North will) carry it out soon. It is expected to take another three months (to complete preparations for a third test)," the source said.
North Korea conducted its first nuclear test in October 2006 and a second in May 2009 in Punggye-ri in the northeastern province of North Hamgyong, the month after it walked out of six-party nuclear disarmament talks.
The Chosun report came as Seoul is preparing to host a Group of 20 summit next month, welcoming world leaders including US President Barack Obama.
North Korea said on Saturday it was willing to resume the six-nation disarmament talks but would not be "hasty" because the United States and some other parties were "not ready".
The United States says the North must mend relations with the South and show sincerity about nuclear disarmament before any resumption of the negotiations.
China, the North's sole major ally and economic lifeline, is pressing to restart the six-party forum, which groups the two Koreas, the United States, China, Japan and Russia and began in 2003.
Prospects for renewed negotiations have been clouded by South Korean and US accusations that the North torpedoed one of Seoul's warships in March, a charge Pyongyang denies.
The 1950-53 Korean War ended only in an armistice and without a formal peace treaty.