As it confirmed it had voided the armistice ending the war with South Korea, the North warned that the next step was an act of "merciless" military retaliation against its enemies.

As it confirmed it had voided the armistice ending the war with South Korea, the North warned that the next step was an act of "merciless" military retaliation against its enemies.

Pyongyang stressed that the real "warmongering" was coming from the United States and its "puppets" in South Korea, a statement by the North's armed forces ministry said.

"They would be well advised to keep in mind that the armistice agreement is no longer valid and (North Korea) is not restrained by the North-South declaration on non-aggression," a ministry spokesman said, according to the official Korean Central News Agency.

The North announced last week that it would nullify the 1953 armistice and peace pacts signed with Seoul in protest over joint South Korea-US military maneuvers that began on Monday.

Because the Korean War was concluded with an armistice rather than a peace treaty, the two Koreas have always remained technically at war.

Meanwhile, South Korea’s Yonhap news agency cited a military source as saying that North Korean jet fighters have flown an "unprecedented" number of sorties in recent days in apparent response to a joint South Korean-US military exercise.

The number of sorties by fighter jets and helicopters peaked at 700 on Monday, the same day the "Key Resolve" exercise was launched, the report said.

The volume of flight missions "is seen as unprecedented in scale", the unidentified source said.

Seoul’s Defense Ministry declined to confirm the report, but reiterated that the North's army, navy and air force were carrying out drills ahead of an expected state-wide exercise.

"Key Resolve" is an annual, largely computer-simulated exercise, but still involves the mobilization of more than 10,000 South Korean and 3,500 US military personnel. About 28,500 US troops are stationed in South Korea.