North Korea leader Kim Jong Un said his country needed to rapidly expand its nuclear armament and called US-South Korea military exercises an "obvious expression of their will to provoke war," state media KCNA reported on Tuesday.

South Korea and its ally the United States kicked off joint military drills this week, including testing an upgraded response to heightened North Korean nuclear threats.

Pyongyang has traditionally criticised those joint drills as rehearsals for invasion and in some cases responded with weapons tests, but Seoul and Washington say they are purely defensive.

The exercises were a "clear expression of stand of openly revealing their intention to remain most hostile and confrontational" to North Korea, Kim said during his visit to a navy destroyer, according to KCNA.

He said the security environment required the North to "rapidly expand" its nuclear armament, noting that recent US- South Korea exercises involved a "nuclear element". (Reuters)