Satellite image dated 25 October 2015 of what appears to be a new tunnel under mountains where North Korea conducts nuclear tests.

Satellite photographs from October and early November indicate North Korea is digging a new tunnel for nuclear testing, but there are no signs that a 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 Oct. 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.

Related articles across the web

• US and S Korea discuss strategy of responding to N Korea's missile launches, nuclear tests
• Japan And Kazakh To Facilitate Entry Into Force Of Nuclear Test-Ban Treaty - Analysis
• North Korea says it has no intention of harming the United States
• The US conducted most of its early nuclear tests in the Pacific, but they became too costly
• China increasingly willing to understand need for pressure on N.Korea: White House official
• China Calls On North Korea To Halt Planned Space Launch, Nuclear Test
• US, South Korea Allies Discuss North Korea's Nuclear, Missile Threat
• Inter-Korean cooperation increasingly frail
• Japan, Kazakhstan urged to lead efforts to realize nuclear test ban
• China Calls On North Korea To Halt Planned Space Launch, Nuclear Test