Satellite photographs from October and early November indicate North Korea is digging a new tunnel for nuclear testing, but there are no signs that such a test is imminent, a U.S. research institute said on Wednesday.

A report on 38 North, a North Korea monitoring website run by Johns Hopkins University's School of Advanced International Studies in Washington, said the images showed significant construction since April at Punggye-ri, on North Korea's east coast, where three previous nuclear tests were conducted.

The commercial images showed excavation of a new tunnel in addition to the three others where North Korea has either conducted nuclear tests or excavated tunnels in the past, the report said.

“While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea's ability to conduct additional detonations at Punggye-ri over the coming years if it chooses to do so,” the report said.

On October 30, South Korea's Yonhap news agency quoted a South Korean government source as saying there was active movement of workers and vehicles working on a new tunnel at the site.

The source said this indicated an intention is to conduct a nuclear test "at some point," though this did not appear to be imminent.

North Korea conducted its last nuclear test in 2013, drawing international condemnation including from China, its main diplomatic ally. North Korea is under United Nations sanctions that ban trade that can fund its arms program.

Pyongyang has vowed to continue to conduct nuclear tests and to launch what it says is a rocket to put a satellite into orbit, something South Korea and its main ally, the United States, say would be a disguised long-range missile test.

Early this month, the United States and South Korean defense chiefs expressed "grave concern" over North Korea test plans and urged it to cease all activities related to its nuclear program immediately.