North Korea has called the use of its flag during US-South Korean military drills last week a serious and hostile provocation that justifies the growth of Pyongyang's nuclear arms programme.

The statement from an unidentified foreign ministry spokesman came on the 62nd anniversary of the start of the Korean war, which ended in a truce, leaving the Korean peninsula still technically at war.

Animosity between North and South Korea, as well as between Pyongyang and Washington, has deepened since a North Korean rocket launch in April. Seoul and Washington called the launch a cover for a test of banned long-range missile technology. North Korea said the rocket, which broke apart shortly after liftoff, was designed to put a satellite into orbit.

Pyongyang has since threatened to attack Seoul's conservative government and media if it doesn't receive an apology for perceived insults against the country and its leader, Kim Jong-un.

Friday's drills were the allies' biggest since the Korean war. South Korean military officials called them a warning to North Korea. The drills took place in Pocheon, close to the border between the two countries. A North Korean flag positioned on a hillside disappeared behind flames and smoke as South Korean jets and US helicopters fired rockets during the drill. In a statement issued on Monday, state-run Korean Central News Agencysaid: "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." The foreign ministry spokesman called North Korea's nuclear programme "an all-powerful treasured sword for preventing a war and reliably protecting peace and stability on the Korean peninsula".

The statement added that North Korea "will further bolster up its nuclear deterrent for self-defence as long as the US, the world's biggest nuclear weapons state, persists in its hostile policy".

Friday's drills coincided with several days of joint naval exercises involving the nuclear-powered aircraft supercarrier USS George Washington and separate US, 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, the South Korean prime minister, Kim Hwang-sik, said his country must "focus on strengthening our national defence and security awareness in order to prevent another Korean war from happening again".