Researchers at a U.S.-based institute have said that satellite photographs from October and early November indicate that North Korea is digging a new tunnel for nuclear testing, but added that it was not a sign of an imminent nuclear test. Work at the Punggye-ri nuclear zone was studied by 38 North, a website run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies, the BBC reported.

The report said that the images show that North Korea can now conduct “additional detonations” if it wants. “Imagery from October and November 2015 show an additional structure and what appear to be significant tailings, indicating excavation of a new tunnel is underway,” the report by 38 North said, adding: “While some analysts conclude each entrance connects to a single tunnel, it is possible that each portal is an entrance to an underground complex capable of supporting multiple nuclear detonations in branches off a main test tunnel.”

The report also added that the latest excavation, adding to three other digs in the region, shows that the country may have conducted nuclear tests, or may have excavated tunnels in the past.

The latest update comes after reports said late last month that the country unsuccessfully tested a submarine-launched ballistic missile -- reports that Pyongyang has yet to confirm. The test followed reports last month that South Korean officials were “closely watching” the North for any hints toward a nuclear test.

In October, South Korean news agency Yonhap said, according to Reuters, that there was an active movement of workers and vehicles at the site and suggested that they were working on a new tunnel. Yonhap cited a source from the South Korean government, who added that this showed that the country is willing to conduct a nuclear test “at some point.”

In September, North Korea announced that the country’s Yongbyon nuclear facility, which was shut down in 2007, had restarted. In October, leader Kim Jong Un called on authorities to boost the country's nuclear arsenal, despite a United Nations ban.