SEOUL • 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 United States-based monitoring group said, citing satellite images.

The 38 North group, run by Johns Hopkins University's School of Advanced International Studies, said yesterday there was 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, referring to the last nuclear test.

The North is said to be ready for another nuclear test at any time and there has been speculation it could mark the Oct 10 anniversary of the founding of its Workers' Party with another underground detonation.

South Korea's Unification Ministry spokesman Jeong Joon Hee told a briefing yesterday there was no particular indication of a plan for a nuclear test timed to coincide with the Oct 10 anniversary.

But a South Korea official told reporters yesterday that the Seoul- Washington alliance believes the North to be preparing for a long- range rocket launch around Monday.

The official said there has been increased activity around the missile launch site in Dongchang-ri, located in the country's north-west.

South Korea President Park Geun Hye's spokesman said the government is prepared for any provocation from the North.

"In case of a nuclear test, (the North) is ready to carry it out whenever they want. The government is keeping an eye on the situation and is prepared to respond to anything," said spokesman Jung Youn Kuk.

North Korea conducted its first nuclear test in 2006 and has, since then, defied United Nations sanctions and rejected international talks to press ahead with the development of the weapons and missiles to carry them, which it says it needs for its defence.

In January, it conducted its fourth nuclear test and last month, its fifth and biggest, on the anniversary of the nation's founding.

The US and South Korea are pushing for tighter sanctions against North Korea by closing loopholes left in a UN Security Council resolution in March.

Last month, the North said it had successfully conducted a ground test of a new rocket engine that would be used to launch satellites. South Korea said the engine could be used for a long-range missile.

North Korea last month fired three missiles that flew about 1,000km. In August, it tested a submarine-launched ballistic missile that international experts said showed considerable progress.

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 situation," Japan's Chief Cabinet Secretary Yoshihide Suga told a news conference.

REUTERS, KOREA HERALD/ASIA NEWS NETWORK