North Korea is digging a new tunnel at its nuclear test site with an eye to conducting more tests of atomic devices in the future, a South Korean news report said on Friday, two days before the leaders of the South, Japan and China meet in Seoul.

US Defense Secretary Ashton Carter also makes a separate visit on Sunday to discuss response to the North's missile and nuclear threat with South Korean defense officials.

The site is on North Korea's east coast where three previous nuclear tests were conducted, and there's an active movement of workers and vehicles working on a new tunnel, Yonhap news agency quoted an unnamed government source as saying.

"The fact that they are constructing a new tunnel indicates the intention is to conduct a nuclear test at some point," the source was quoted as saying. There was no evidence to conclude the preparation was for an imminent test, the source added.

South Korea's Unification Ministry spokesman Jeong Joon-hee declined to confirm the report but said the country and the United States are closely watching for any nuclear activity by the North.

The report comes as the leaders of South Korea, Japan and China are scheduled to meet in Seoul on Sunday where reigning in the North's pursuit of weapons of mass destruction is likely to be discussed.

North Korea has been steadily working on its nuclear program, but a fourth nuclear test was not see as imminent, particularly after it agreed with South Korea in August to work toward easing tensions on the peninsula and improve ties.

The North has conducted three nuclear tests, the last in 2013, drawing international condemnation including from China, its main diplomatic ally, and is under UN sanctions that ban trade that can fund its arms programs.

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