SEOUL • Satellite imagery indicates a high level of activity at North Korea's nuclear test site to ensure the facility is always ready for use on any orders from Pyongyang, a United States think-tank has said.

The US-Korea Institute at Johns Hopkins University said images from July 7 of the Punggye-ri site show what appear to be supplies and/or equipment stacked next to the North Portal where North Korea carried out its fourth nuclear test in January.

A small vehicle was at the nearby support building and several mine ore carts might be present south-east of the portal, suggesting work was being carried out at the tunnel.

"Based on imagery alone, it is not possible to determine whether this activity is for maintenance, excavation or preparation for a fifth nuclear test," the institute said on Monday on its website 38 North.

"Nevertheless, it is clear that North Korea is ensuring that the facility is in a state of readiness that would allow the conduct of future nuclear tests should the order come from Pyongyang," it said.

Tensions are high since North Korea carried out its nuclear test in January, followed by a series of missile launches.

Pyongyang on Monday threatened to take "physical action" after Washington and Seoul announced they would deploy a sophisticated US anti-missile defence system known as the Terminal High Altitude Area Defence (Thaad) system in South Korea to counter Pyongyang's threat.

Citing an unnamed inside source, Yonhap news agency reported that the Thaad system will be hosted in Seongju county's Seongsan-ri region, about 300km south of Seoul, where population is sparse.

An announcement is likely to be made as early as next week.

AGENCE FRANCE-PRESSE