WASHINGTON (Reuters) – Satellite images show that North Korea may have resumed tunnel excavation at its main nuclear test site, similar to activity seen before the country’s most recent nuclear test in January, a U.S. North Korea monitoring website reported on Wednesday.

The 38 North website, run by the Johns Hopkins University’s School of Advanced International Studies, said such activity could be carried out as part of preparations for a nuclear test, as was the case in January, or to conceal such preparations.

It said commercial satellite images of the West Portal of the Punggye-ri test site taken on Tuesday showed two small ore carts on a track crossing a road from a tunnel entrance.

“The presence of the two carts … and the absence of any notable changes in the spoil pile suggests that tunnel excavation operations are about to resume, or have recently resumed, for the first time this year,” the 38 North report said.

The report said the images also showed limited movement of vehicles and equipment at the site’s North Portal, where the past three North Korean nuclear tests took place, compared to images taken on April 14.

“These activities by themselves do not establish that test preparations are imminent. However, the possibility of an impending test cannot be ruled out,” the report said.

“Pyongyang has clearly demonstrated, with its fourth nuclear detonation this past January, the ability to conduct detonations on short notice while masking indicators of its preparations from satellite view.”

A South Korean official declined to comment on any new intelligence reports of activities at the site but said the military was on high alert over the possibility that the North could conduct a nuclear test at any time.

North Korean leader Kim Jong Un said in March that his country has miniaturized a nuclear warhead and ordered tests of a nuclear warhead and ballistic missiles in defiance of U.N. sanctions.

38 North reported in early December that satellite photographs from the two previous months indicated North Korea was digging a new tunnel for nuclear testing.

North Korea conducted its fourth nuclear test on Jan. 6 and has vowed to conduct more, despite the stepped up international sanctions.

Some experts expect North Korea to conduct a fifth nuclear test before a ruling party congress in early May, following an embarrassing failure in the test of an intermediate-range missile last week.

The top U.S. diplomat for the Asia-Pacific region warned on Tuesday that a fifth North Korean nuclear test could trigger new sanctions including an effort to choke off hard currency earnings by its workers abroad.

(Reporting by David Brunnstrom; additional reporting by Jack Kim in Seoul; Editing by Tom Brown and Raju Gopalakrishnan)