South Korea urged North Korea 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 North Sunday fired the volley of rockets into the Sea of Japan (East Sea), the latest in a series of launches in recent weeks that have sparked criticism from Seoul and Washington.

The show of force is apparently intended to express anger at the South's continuing joint military exercises with its ally the United States.

"The North should stop actions that cause military tension and unnerve its neighbours," Seoul's defence ministry spokesman Kim Min-Seok told reporters.

"Provocative action made without any prior notifications... can pose significant danger to sea vessels and aircraft passing by the area," he added.

The South's military was closely watching the North's troop movements, Kim said, citing the possibility of more rocket launches.

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

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

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

China's special envoy Wu Dawei arrived in Pyongyang Monday, the North's official Korean Central News Agency (KCNA) said in a brief report which did not specify the trip's purpose.

The annual South Korean-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 military drills south of the border -- as rehearsals for an invasion.

Seoul and Washington say they are purely defensive.

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

But Seoul's defence ministry said 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, KCNA said Monday.

The servicemen vowed to bring down "robber-like US imperialists" when ordered by Kim as he praised their combat-readiness in the spirit of becoming "human bombs", it said.

Separately, Kim led a meeting of the ruling party's Central Military Commission to discuss combat-readiness and "important matters arising in increasing defence capability", the news agency said.

The meeting also discussed the military's "organisational issue", it said, suggesting possible personnel changes aimed at strengthening the young ruler's grip on the armed forces.

Former members of the commission included Jang Song-Thaek, Kim's once-powerful uncle who was executed last December for charges including treason.

Hyon Yong-Chol, who failed to secure a seat in this month's parliamentary election, was also a member of the commission. But he is believed to have been replaced -- or to be about to be replaced -- after being dismissed as military chief last year.