The statement from an unidentified Foreign Ministry spokesman came on the 62nd anniversary of the start of the 1950-53 Korean War, which ended in a truce, leaving the Korean Peninsula still technically at war.
Animosity between the rival Koreas and between Pyongyang and Washington has deepened since a North Korean rocket launch in April that Seoul and Washington called a cover for a test of banned long-range missile technology. North Korea says the rocket, which broke apart shortly after liftoff, was meant to put a satellite into orbit.
Pyongyang has since threatened to attack Seoul's conservative government and media if it doesn't get an apology for perceived insults against the country and its new, young leader, Kim Jong Un.
The U.S.-South Korean drills Friday were the allies' biggest since the Korean War, and South Korean military officials called them a warning to North Korea. A huge North Korean flag on a hill disappeared behind flames and smoke as South Korean jets and U.S. helicopters fired rockets. The flag wasn't hit.
"It is an extremely grave military action and politically-motivated provocation to fire live bullets and shells at the flag of a sovereign state without a declaration of war," said the North's statement, which was dated Sunday but was released by the official Korean Central News Agency early Monday.
The Foreign Ministry spokesman called the North's nuclear program "an all-powerful treasured sword for preventing a war and reliably protecting peace and stability on the Korean Peninsula."
North Korea "will further bolster up its nuclear deterrent for self-defense as long as the U.S., the world's biggest nuclear weapons state, persists in its hostile policy."
The drills Friday coincided with several days of joint naval exercises involving the nuclear-powered aircraft supercarrier USS George Washington and separate U.S., South Korean and Japanese naval rescue drills. On Sunday, F-18 flights arrived and departed every few minutes on the carrier as a light drizzle fell over choppy seas.
During a ceremony in Seoul on Monday, South Korean Prime Minister Kim Hwang-sik said his country "must focus on strengthening our national defense and security awareness in order to prevent another Korean War from happening again."