By Jeremy Laurence
SEOUL (Reuters) - A U.S. satellite has detected increased activity at a North Korean nuclear weapons test site, suggesting it could be preparing for a third test, a South Korean government source was quoted as saying on Thursday.
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.
Defense and foreign ministry officials could not confirm the report in the Chosun Ilbo daily about activity at the test site. The U.S. State Department declined to comment on what it called an intelligence matter, but warned Pyongyang not to engage in any provocative behavior.
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 U.N. 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 last month by his ailing father Kim Jong-il, might now try to burnish a hardline image by carrying out an act of brinkmanship.
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, scrutinized by U.S. 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.
The State Department said on Thursday it could not comment in detail on intelligence matters and indicated it had no information to back up the specific report. But it also took the opportunity to caution Pyongyang.
"Our message to North Korea has been clear and consistent: It needs to cease its provocative actions. And I can think of nothing more provocative than the possibility of another nuclear test or another series of missile firings. It's the last thing that North Korea should consider at this point," State Department spokesman P.J. Crowley said.
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.
TEST NOT IMMINENT
The Chosen Ilbo daily quoted an unnamed source as saying the reconnaissance satellite had recently detected "brisk movement" of vehicles and people 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 was being repaired.
However, it seems unlikely 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.
North Korean Vice Foreign Minister Pak Kil-yon told the U.N. General Assembly last month that Pyongyang would bolster its "nuclear deterrent" in response to the threat posed by the United States, but promised never to use its atomic arsenal to attack or threaten any nation.
(Additional reporting by Danbee Moon, and Andrew Quinn in Washington; Editing by David Chance and Jerry Norton)