Photographs showed Kim and his daughter, both wearing matching black jackets, accompanied by uniformed officers as they watched an artillery unit fire a volley of missiles on Thursday.

The South Korean military said yesterday it had detected the launch of one ballistic missile and was analysing the possibility of multiple launches from the same location.

Photographs released Friday by the official Korean Central News Agency (KCNA) showed at least six missiles being fired at the same time by the Hwasong unit, which it said is trained for "strike missions".

The unit "fired a powerful volley at the targeted waters in the West Sea of Korea", KCNA said.

While inspecting the drills, Kim told soldiers to be prepared for "two strategic missions, that is, first to deter war and second to take the initiative in war", according to KCNA.

Such units should be "steadily intensifying various simulated drills for real war in a diverse way in different situations", he added.

The drill came as South Korea and the United States prepared to kick off on Monday their largest joint exercises in five years.

Relations between the two Koreas are at one of their worst points in decades, with the nuclear-armed North conducting ever more provocative banned weapons tests while the South ramps up security cooperation with Washington in response.

- Attack air bases -

The drill marked another high-profile appearance for Kim's daughter, who is believed by South Korean intelligence to be his second child, named Ju Ae.

She is regarded by some experts as Kim's de facto heir. North Korean media have only identified her as his daughter, and not released any other details, including her name.

Believed to be around 11 years old, she was seen sitting near Kim as he inspected the drill.

"It looks like Ju Ae's appearance in events related to the North's nuclear or missile developments... has been regularised," Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP.

Pyongyang has long claimed its nuclear weapons and missile programmes are for self-defence, while bristling over US-South Korea military exercises, describing them as rehearsals for an invasion.

KCNA said the Thursday drill was set "under the simulated conditions of the major elements of the enemy operation airport".

North Korea's air force is the weakest link in its military, and the exercise highlights the strategy to compensate for that, experts said.

"North Korea's latest drills, like many of its previous ones, has the purpose of blocking South Korean (warplanes) from taking off," An Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.