North Korea said Friday it had tested a purported underwater nuclear attack drone in response to a combined naval exercise between South Korea and the United States and Japan this week, as it continues to blame its rivals for raising tensions in the region.

The alleged drone test came days after North Korean leader Kim Jong-un declared he would scrap his country's long-standing goal of a peaceful unification with South Korea and that his country would rewrite its constitution to define South Korea as its most hostile foreign adversary.

Tensions on the Korean Peninsula are at their highest point in years with Kim accelerating weapons demonstrations and threatening nuclear conflict and the US and its Asian allies responding by strengthening their combined military exercises.

The test was not of a nuclear device but rather of an "underwater self-explosive drone," according to Shin Jong-woo, a military expert at the Seoul-based Korea Defense and Security Forum. 

Shin called it a "navigation test for their battery-powered underwater suicide drone," adding North Korea was testing how the drone would perform over long distances and long periods of time.

Shin noted North Korea tested the Haeil-2 Unmanned Underwater Nuclear Attack Boat between April 4 and 7 of last year. 

North Korea's alleged nuclear attack drone is among a broad range of weapon systems demonstrated in recent years as Kim expands his arsenal of nuclear-capable weapons. South Korea's military has insisted the North has exaggerated the capabilities of the drone, which is supposedly designed to carry out strikes on enemy vessels and ports.

The North's military said it conducted the test in the country's eastern waters in response to the U.S., South Korean, and Japanese naval drills, which wrapped up its three-day run Wednesday in waters south of Jeju island.

"Our army's underwater nuke-based countering posture is being further rounded off and its various maritime and underwater responsive actions will continue to deter the hostile military maneuvers of the navies of the U.S. and its allies," the North's Defense Ministry said in a statement.

"We strongly denounce the U.S. and its followers for their reckless acts of seriously threatening the security of the DPRK from the outset of the year and sternly warn them of the catastrophic consequences to be entailed by them."