North Korea on Monday called the use of its flag during U.S.-South Korean military drillslast week a serious provocation and evidence of U.S. hostility that justifiesthe growth of Pyongyang’s nuclear arms program.

The statement from an unidentified Foreign Ministry spokesman came on the 62ndanniversary 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 deepenedsince a North Korean rocket launch in April that Seoul and Washington called acover for a test of banned long-range missile technology. North Korea says therocket, which broke apart shortly after liftoff, was meant to put a satelliteinto orbit.

Pyongyang has since threatened to attack Seoul’s conservative government and media if itdoesn’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. Ahuge North Korean flag on a hill disappeared behind flames and smoke as SouthKorean jets and U.S. helicopters fired rockets. The flag wasn’t hit.

“It is an extremely grave military action and politically-motivated provocation tofire live bullets and shells at the flag of a sovereign state without adeclaration of war,” said the North’s statement, which was dated Sunday but wasreleased by the official Korean Central News Agency early Monday.

The Foreign Ministry spokesman called the North’s nuclear program “an all-powerfultreasured sword for preventing a war and reliably protecting peace andstability on the Korean Peninsula.”

North Korea “will further bolster up its nuclear deterrent for self-defense as longas the U.S., the world’s biggest nuclear weapons state, persists in its hostilepolicy.”

The drills Friday coincided with several days of joint naval exercises involvingthe nuclear-powered aircraft supercarrier USS George Washington and separateU.S., South Korean and Japanese naval rescue drills. On Sunday, F-18 flightsarrived and departed every few minutes on the carrier as a light drizzle fellover choppy seas.

During a ceremony in Seoul on Monday, South Korean Prime Minister Kim Hwang-sik saidhis country “must focus on strengthening our national defense and securityawareness in order to prevent another Korean War from happening again.”