More than 800,000 young North Koreans have volunteered to join the army to fight "US imperialists", state media said Saturday, days after Pyongyang test-fired its most powerful intercontinental ballistic missile.

After a record-breaking year of weapons tests and growing nuclear threats from Pyongyang, Seoul and Washington have ramped up security cooperation, and this week kicked off their largest joint military drills in five years.

North Korea views all such exercises as rehearsals for invasion and has repeatedly warned it would take "overwhelming" action in response.

The official Korean Central News Agency described the ongoing drills as an American attempt "to provoke a nuclear war" and said that in response, hundreds of thousands of people had enlisted.

The young volunteers are determined to "mercilessly wipe out the war maniacs" so they joined the army to "defend the country", KCNA said.

"More than 800 000 youth league officials and students across the country volunteered to join and rejoin the Korean People's Army" on Friday alone, it added.

Images released by Pyongyang's official Rodong Sinmun showed young North Koreans waiting in long lines to sign their names at what looks like a construction site.

The latest report comes after Pyongyang test-fired its largest and most powerful missile, a Hwasong-17, on Thursday -- its second ICBM test this year.

State media has described the launch as a response to the ongoing, "frantic" US-South Korea joint military drills.

On Saturday, KCNA said the ongoing drills were "inching close to the unpardonable red-line".

Last year, North Korea declared itself an "irreversible" nuclear power, and leader Kim Jong Un recently called for an "exponential" increase in weapons production, including tactical nukes.

Kim earlier this month also ordered the North Korean military to intensify drills to prepare for a "real war".

Pyongyang is using the drills to justify their nuclear weapons program domestically as "crucial and necessary," Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP.

This involves "spreading the idea that the South Korea-US military drills ultimately aim to destroy the current North Korean regime and even occupy its capital Pyongyang," Yang added.