SEOUL (AFP) - South Korea called on North Korea on Monday to stop what it called "provocative" and potentially dangerous rocket and missile tests, a day after Pyongyang test-fired 25 projectiles into the sea.

The rockets were fired into the Sea of Japan (East Sea), the latest in a series of launches that have sparked criticism from Seoul and Washington.

The North carried out a series of missile and rocket tests between late February and mid-March in what is seen as a show of force to express anger at the South's ongoing joint military exercises with the United States.

"The North should stop actions that cause military tension and unnerve its neighbours," Seoul's defence ministry spokesman told reporters, adding that "provocative action made without any prior notifications...can pose significant danger to sea vessels and aircraft passing by the area".

Beijing expressed concern earlier this month after North Korea test-fired a rocket into the flight path of a Chinese airliner.

The South's military was closely watching the North's troop movements, said the spokesman, adding there was a possibility of more rocket launches.

They were ageing versions of Russian-developed Frog rockets, he said, noting that the North on Sunday fired "more rockets than normally expected".

The US State Department on Sunday called on Pyongyang to refrain from "provocative actions that aggravate tensions".

The annual South Korea-US military drills started in late February and will run until mid-April.

The North has habitually slammed the Key Resolve and Foal Eagle exercises - along with other army drills south of the border - as rehearsals for an invasion.

Seoul and Washington say they are purely defensive.

The North's powerful National Defence Commission on Friday threatened to demonstrate its nuclear deterrent in the face of what it called US hostility.

Seoul's defence ministry said however there was no sign of an imminent nuclear test by the North, which staged three atomic tests in 2006, 2009 and last year.

As the North continues to flex its military muscle, its leader Kim Jong Un guided an air force and air defence exercise, the state news agency KCNA said on Monday.

The servicemen vowed to bring down "robber-like US imperialists" when ordered by Mr Kim, it said.

He praised their combat readiness in the spirit of becoming "human bombs", it added.

He also led a key meeting to discuss the military's "organisational issue", KCNA said, suggesting possible personnel changes aimed at strengthening the young ruler's grip on the armed forces.

The meeting of the Central Military Commission of the ruling Workers' Party also discussed combat readiness and "important matters arising in increasing defence capability", the news agency said without elaborating.

Members of the powerful commission chaired by Mr Kim previously included Jang Song Thaek, his once-powerful uncle who was executed last December for charges including treason.

Mr Hyon Yong Chol was also a member of the commission but is believed to have been replaced - or about to be replaced - after being dismissed as military chief last year.

He failed to secure a seat in this month's state-managed elections to parliament, suggesting he had fallen further out of favour.