Leader Kim Jong Un was at the exercise, which took place on Wednesday, the Korean Central News Agency (KCNA) said.

The missiles flew for 130 minutes and travelled at a 1,587 kilometre-long (986 mile-long) trajectory before they "precisely hit the targets", according to KCNA.

Pyongyang said the drills were aimed at warning nuclear-armed North Korea's enemies of its "counterattack capability in any space and the readiness of its various nuke operation means".

Images in North Korean state media showed Kim, flanked by officials, sporting binoculars.

A missile is also shown destroying a small building, which erupts in flames.

Attending the drill, Kim said it was the responsibility of North Korea's nuclear forces to "defend the national sovereignty and security".

"Powerful striking ability", he said, served as "the most perfect deterrence and defence", KCNA reported.

Pyongyang accused North Korea's enemies of "seriously violating the security environment" as well as "fostering and escalating" confrontation.

KCNA did not say where the test took place, but specialist website NK News said it likely took place near the city of Nampho, roughly 130 kilometres (80 miles) from the border with South Korea.

Seoul's military said it had "tracked and monitored North Korea's launch of several cruise missiles" on Wednesday.

"Our military is closely monitoring various North Korean trends... while maintaining the ability and posture to overwhelmingly respond to any provocation," the Joint Chiefs of Staff said.

Analyst Yang Moo-jin said the test could have served as a "stepping stone to check the readiness for nuclear operation".

Pyongyang could "gradually increase the level and frequency of provocations in the future", said Yang, the president of the University of North Korean Studies.

- 'Modern warfare' -

Relations between Pyongyang and Seoul have been at one of their lowest points in years, with the North launching a flurry of ballistic missiles last year in violation of UN sanctions.

Joint South Korea-US "Freedom Shield" military exercises are set to kick off next month, Seoul said this week.

Those drills enrage Pyongyang, which claims they are a rehearsal for an invasion and often carries out missile tests in response.

The two Koreas remain technically at war since the 1950-1953 conflict ended in an armistice, not a peace treaty.

North Korea last conducted such a test in January, when it said it had launched sea-to-surface strategic guided cruise missiles -- Pyongyang's first since US President Donald Trump returned to the White House.

Ahn Chan-il, a defector and researcher who runs the World Institute for North Korea Studies, told AFP this week's launch could be a bid for attention from the US President.

"North Korea wants to have a summit with Trump this year, so this could be their signal to press the new US president by test-launching a strategic weapon," he said.

US and South Korean intelligence also believe that North Korea has sent thousands of troops to Russia to fight against Ukraine and has since suffered hundreds of casualties.

On Thursday, South Korean intelligence told AFP that Pyongyang had dispatched more troops to Russia, though it could not give a precise figure.

It also said North Korea had redeployed soldiers to the frontline in Kursk, where Ukraine previously said they had been withdrawn following heavy losses.

This week, Kim visited a major military academy, urging his troops to harness the "actual experiences of modern warfare", according to state media.

Neither North Korea nor Russia has officially confirmed that Pyongyang's forces are fighting for Moscow.