The drills were "seriously threatening the security" of the North, so in response, Pyongyang "conducted an important test of its underwater nuclear weapon system 'Haeil-5-23' under development in the East Sea of Korea," according to a statement from the defense ministry carried by state news agency KCNA.
Earlier this week, South Korea, the United States and Japan carried out joint naval drills in waters off southern Jeju Island, which they said were in response to North Korea's Sunday launch of a hypersonic missile.
The drills involved nine warships from the three countries, including the aircraft carrier USS Carl Vinson.
Pyongyang said the drills "constituted a cause of further destabilizing the regional situation, and they are an act of seriously threatening the security" of the North.
North Korean leader Kim Jong Un has upped the rhetoric in recent weeks. In late December, he said Pyongyang would not hesitate to respond with a nuclear attack when an enemy provokes it with nuclear weapons.
Days later, Kim told the country's military commanders that the most powerful means must be mobilized to destroy the United States and South Korea if they choose a military confrontation.
Last week, the North Korean leader declared the South his country's "principal enemy", jettisoned agencies dedicated to reunification and outreach and threatened war over "even 0.001 mm" of territorial infringement.
