North Korea revealed the “‘Haeil-2’ underwater nuclear attack drone” for the first time Saturday, reporting the system was tested earlier this week to “deter all sorts of escalated military actions of the enemy.”

It is the third test of the new category of nuclear weapon to be disclosed in state media. In a report unveiling the “Haeil” system last month, Pyongyang said it had been in development since 2012 and undergone dozens of tests in secret.

The test comes as North Korea has continued to protest ongoing U.S.-South Korea military drills, and as Japan joined the two for naval