A US satellite has reportedly detected increased activity at a North Korean nuclear weapons test site, suggesting it could be preparing for a third test.
The report comes after satellite images taken last month also showed heightened activity at the North's main Yongbyon nuclear complex, indicating Pyongyang was pushing ahead with its nuclear plans in defiance of international pressure.
Defence and foreign ministry officials could not confirm the report about activity at the test site in the Chosun Ilbo newspaper, which cited South Korean security sources.
North Korea says its wants to rejoin international nuclear disarmament talks after a two-year boycott, which analysts said was an indication it was hurting badly under harsh UN sanctions imposed last year in response to its second nuclear test.
Experts worry the North's young leader-in-waiting, Kim Jong-un, appointed to senior military and political posts by his ailing father, Kim Jong-il, last month, might now try to burnish a hardline image by carrying out an act of brinksmanship.
Previous nuclear tests, trumpeted at home, have been used by incumbent leader Kim Jong-il to boost his stature with his country's powerful armed forces and rally the masses around his guiding military-first rule.
North Korea, scrutinised by US spy satellites, can easily signal it is preparing for a test by moving equipment but that does not mean a blast is imminent.
In April, Washington dismissed a similar report that the secretive North was readying for a third test. A third test would improve North Korea's ability to make nuclear weapons but also decrease its supply of fissile material, thought to be enough for six to eight nuclear bombs, experts say.
The Chosun Ilbo daily quoted an unnamed source as saying "brisk movement" of vehicles and people had been recently detected by the reconnaissance satellite in Punggye-ri, the location of the North's past tests in 2006 and 2009.
The source said there were also signs a tunnel that collapsed after the two earlier nuclear tests is being repaired.
However, it seems unlikely that the North will conduct a third test in the immediate future since current activities there suggest it will take "about three months" to prepare, the source added.