The US military shot down a dummy warhead over the Pacific on Friday in what it called a "huge step" in the development of an anti-missile shield activated in July to guard against missiles test-fired by North Korea.
The North's Committee for Peaceful Reunification of the Fatherland said the joint exercise was the most provocative yet and, given its scale and content, a virtual declaration of war against the communist state.
The committee's statement on Saturday, carried by the official KCNA news agency, read: "On the occasion of the Ulchi Focus Lens joint military drills, the United States even conducted a missile test aimed at attacking us from the South and the US homeland and intercepting our missiles."
On Friday, US and South Korean troops ended the 12-day exercise designed to test command structures and communications.
Ulchi Focus Lens has been staged annually without incident since 1975, but the North usually denounces the drills as a prelude to invasion and nuclear war.
This year's exercise took place with tensions high on the peninsula after North Korea test-fired a barrage of missiles on July 5 and after reports in August that it might be preparing to test a nuclear weapon.
Pyongyang said last year that it now possessed nuclear arms, which it described as a self-defence deterrent. There has been no outside confirmation of this development.
"The United States are wielding the bat of power and trying to get someone to surrender to them ... it will cement the determination of our armed force and people to build up self-defence deterrent," Saturday's statement said.