SEOUL (Reuters) - An increase in activity at North Korea's nuclear test site could signal preparations for a new test or a collection of data from its last one, a U.S.-based monitoring group said on Friday.
The 38 North group, run by Johns Hopkins University's School of Advanced International Studies, said satellite images showed activity at all three tunnel complexes at the Punggye-ri nuclear test site involving a large vehicle and personnel.
"One possible reason for this activity is to collect data on the Sept. 9 test, although other purposes cannot be ruled out, such as sealing the portal or other preparations related to a new test," the group said.
North Korea conducted its fifth and biggest nuclear test on Sept. 9 and South Korea has said it believes the north is ready to conduct another nuclear test at any time. There has been speculation that Pyongyang could mark the Oct. 10 anniversary of the founding of its Workers' Party with a sixth detonation.
A study published on Friday by a U.S.-based North Korea research project said North Korean missile and nuclear tests, and other major "provocations", had clustered increasingly closer to U.S. elections.
The study from Washington's Center for Strategic and International Studies said the pattern based on looking at 30 U.S. elections since 1956 suggested a North Korean action as early as a month before the Nov. 8 U.S. presidential election.
This could mean a test coinciding with the Oct.10 anniversary. The trend also suggested the possibility of an act during the December transition period for the next U.S. administration.
North Korea conducted its first nuclear test in 2006 and has since defied U.N. sanctions to press ahead with the development of the weapons and missiles to carry them, which it says it needs for defence.
In January, it conducted its fourth nuclear test and the fifth was carried out on the anniversary of the nation's founding.
South Korea's Unification Ministry spokesman Jeong Joon-hee told a briefing there were no particular indications of a plan for a nuclear test on Oct. 10.
However, South Korea's Yonhap news agency cited an unidentified government official as saying that activity at North Korea's rocket launch station near the west coast could be preparations for a long-range missile launch.
Japan said the possibility of further "provocative action" by North Korea could not be ruled out.
"The government is taking all possible measures in gathering information, exercising vigilance and taking surveillance activities to be able to respond to any situations," Japan's Chief Cabinet Secretary Yoshihide Suga told a news conference.
Reporting by Jack Kim and Ju-min Park in Seoul and David Brunnstrom in Washington; Editing by Robert Birsel and James Dalgleish