North Korea appears to have conducted a secret nuclear test in 2010 and the explosion could have gone unnoticed by the outside world, scientists have claimed, adding to a series of similar allegations.

   Researchers Michael Schoeppner of Princeton University and Ulrich Kuhn of the University of Hamburg made the claim in an article on the website of the Bulletin of the Atomic Scientists, citing a seismic analysis published last month.

   In May 2010, multiple radionuclide and noble gas stations in East Asia picked up traces of various radionuclides that have been interpreted as a release from a nuclear test. But some of those radionuclides have never been detected before in the history of the International Monitoring System, and the possible source became the subject of controversy, they said.

   "Seismic analysis published in January 2015 seems to support the idea that it was a nuclear test. But it's important to note that the radionuclide-noble gas component of the monitoring system detected something contemporaneously that the waveform detection methods missed at that time," they said.

   It wasn't the first time that the North has been alleged to have conducted a nuclear test in 2010.

   In November, Wen Lianxing, a professor of the Beijing-based University of Science and Technology of China, claimed that North Korea conducted a "low-yield underground nuclear test" in May 2010 and the test yielded a 2.9-ton blast with a margin of error of 0.8 ton.

   In 2012, Lars-Erik De Geer, an atmospheric scientist with the Swedish Defense Research Agency, claimed in a military science journal that the North conducted two nuclear tests in April and May of 2010, citing analysis of atmospheric samples collected from over the Korean Peninsula.

   But these claims have not been officially confirmed.

   The official record is that the North has so far conducted three nuclear tests, in 2006, 2009 and 2013. (Yonhap)