North Korean leader Kim Jong-Un has urged his army to prepare for war with the United States and its allies, state media said Saturday, as Pyongyang ramps up the rhetoric ahead of US-South Korea military drills.

Kim's comments came after South Korea and the United States Friday conducted a joint naval drill involving 10 South Korean warships and a US Aegis destroyer, ahead of the launch of large-scale military exercises that have enraged the North.

"The prevailing situation where a great war for national reunification is at hand requires all the KPA (Korean People's Army) units to become (elite) Guard Units fully prepared for war politically and ideologically, in military technique and materially", he was quoted by the official Korean Central News Agency (KCNA) as saying.

North Korea regularly ratchets up hostile rhetoric at times of joint US-South Korea military exercises that spark a sharp surge in tensions on the divided peninsula.

Kim called on the military to train hard in order "to tear to pieces the Stars and Stripes", in comments made while opening a new hall at the Victorious Fatherland Liberation War Museum in Pyongyang, KCNA said.

The drill Friday was a prelude to an eight-week exercise, Foal Eagle, involving air, ground and naval field training, with around 200,000 Korean and 3,700 US troops that begins on Monday.

A week-long, largely computer-simulated joint drill, Key Resolve, will also get under way.

Seoul and Washington insist the exercises are defence-based in nature, but they are condemned by Pyongyang as provocative rehearsals for invasion.

North Korea had offered a moratorium on carrying out nuclear tests if this year's joint drills were cancelled -- a proposal rejected by Washington as an "implicit threat" to carry out a fourth atomic drill.

North Korea claims it won the 1950-53 Korean War, which ended in an armistice instead of a peace treaty, leaving the two Koreas still technically at war.