Satellite images reveal North Korea may have restarted a tunnel-excavating project at its main nuclear test site signaling that another major launch could be on the way, a key monitoring website says.

The 38 North website, run by Johns Hopkins University's School of Advanced International Studies, said the activity could be carried out as preparations for another test, Reuters reported Thursday. North Korea’s last nuclear test was in January.

The website showed two images of small ore carts on a track crossing a road from a tunnel entrance at the Punggye-ri test site.

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

A South Korean official declined to comment on the new images but said the military is alert over the possibility of more North Korean nuclear tests, Reuters reports. A U.S. diplomat for the region warned Tuesday that future tests could trigger new sanctions against North Korea.

The reclusive country has vowed to conduct more nuclear tests, but suffered an embarrassing setback last week after its new intermediate-range ballistic missile exploded on launch.

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