North Korea claimed Friday to have tested a nuclear-capable underwater drone designed to generate a gigantic “radioactive tsunami” that would destroy naval strike groups and ports. Analysts were skeptical that the device presents a major new threat, but the test underlines the North’s commitment to raising nuclear threats.

The test this week came as the United States reportedly planned to deploy aircraft carrier strike groups and other advanced assets to waters off the Korean Peninsula. Military tensions are at a high point as the pace of both North Korean weapons tests and U.S.-South Korea joint military exercises has accelerated in the past year in a cycle of tit-for-tat responses.