North Korean leader Kim Jong Un has called for an increase in missile production to help secure "overwhelming military power" and be ready for war, state media KCNA said on Monday, as South Korea and the United States gear up for annual military drills.

Kim gave the order as he visited key munitions factories that produce tactical missiles, missile launch platforms, armoured vehicles and artillery shells on Friday and Saturday.

His field inspection was the latest in a string of visits to arms factories, where he ordered mass production of weapons, and came days before South Korea and the U.S. are to begin annual military drills, which Pyongyang sees as a rehearsal for war.

Kim noted an "important goal to dramatically increase" missile production capacity to meet the needs of the expanded and strengthened frontline military units, KCNA said.

"The qualitative levels of war preparations depend on the development of the munitions industry, and the factory has a tremendous responsibility in accelerating our military's war preparations," he was quoted as saying.

At other plants, Kim inspected and drove a new utility combat armoured vehicle, and praised recent progress in modernising production lines for large-caliber multiple rocket launcher rounds, KCNA said.

There was a "very urgent need" to "exponentially increase" the production of such rockets to strengthen frontline artillery units, he said.

"Our army must thoroughly secure overwhelming military power and solid readiness to handle any war at any time, so that the enemy does not dare to use force, and would be annihilated if it does," Kim said.

South Korea and the United States said on Monday that they would stage the Ulchi Freedom Guardian summer exercises on Aug. 21-31 to improve their ability to respond to North Korea's evolving nuclear and missile threats.

North Korea has denounced the allies' military drills as a rehearsal for nuclear war.

This year's drills will be held on the "largest scale ever", mobilising tens of thousands of troops from both sides, as well as some member states of the U.N. Command, for about 30 field training programmes, according to South Korea's Joint Chiefs of Staff (JCS).

"The exercises are an essential element in maintaining robust combined defence posture in case of emergency - absolutely necessary to respond to the growing military threat from North Korea," JCS spokesman Col. Lee Sung-jun told a briefing.

The United States has accused North Korea of providing weapons to Russia for its war in Ukraine, including artillery shells, shoulder-fired rockets and missiles.

North Korea and Russia have denied any arms transactions.

The leaders of South Korea, the United States and Japan are also set to discuss security cooperation over North Korea, Ukraine and other issues when they gather for a trilateral summit on Aug. 18 at Camp David.

KCNA separately said on Monday that Kim visited "typhoon-hit areas" after tropical storm Khanun swept over the Korean peninsula last week, flooding farmlands.