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, citing satellite images.

The 38 North group, run by Johns Hopkins University's School of Advanced International Studies, said 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 believed 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, Reuters informs.

North Korea conducted its first nuclear test in 2006 and has since then defied U.N. 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 defense.

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

The United States and South Korea are pushing for tighter sanctions against North Korea by closing loopholes left in a U.N. Security Council resolution in March.

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.