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.