Xenon, an element that is found in trace amounts after nuclear weapons tests and other nuclear activities, was detected by a South Korean nuclear regulatory expert organization in June, Yonhap News Agency reported.
The Korea Institute for Nuclear Safety detected the presence of the gaseous element three times in June, sources told the wire service.
A nuclear weapons test by North Korea was conducted in February, but no signs of nuclear activity emanated from the North around the time the gas was discovered, according to Yonhap. Xenon isotopes break down relatively quickly, so the detected gas may not be from any recent nuclear activity from the North, according to a source for the Korean wire service. The rare isotopes found over South Korea could have originated in Japan, from radiation leaks stemming from the damaged Fukushima Daiichi nuclear facility, the Wall Street Journal reported.
"Relevant agencies are conducting an analysis, and that's what we know of," a South Korean Defense Ministry spokesman told the Journal.
A North Korean announcement in April that a reactor and uranium enrichment plant at the Yongbyon complex would be activated has not been detected by the South, according to Yonhap's sources, but the facility could have begun the start-up process in May.
Meanwhile, South Korea's special representative for peace and security affairs left the Asian nation for a three-day trip to Russia, where he will meet with Russia's top negotiator for Pyongyang's denuclearization, Yonhap also reported.
"I plan to share detailed opinions on how to assess North Korea's nuclear programs and threats and discuss how to push ahead with efforts to denuclearize the North during this visit," the South Korean envoy told Yonhap.
A seventh round of meetings between the North and the South over the future of the shuttered Kaesong Industrial Complex will begin on Wednesday. Negotiations will hinge on making progress on conditions that South Korea asserts are necessary to prevent work stoppages in the future, Yonhap reported separately on Tuesday.
North Korea is also attempting to improve diplomatic ties with various countries in Africa, in a move that observers say is an attempt to escape the global isolation that the Asian nation has faced during its pursuit of nuclear weapons, according to another Yonhap report.