The actions come one day after the communist state expelled South Korean officials from a joint industrial estate, in protest at the new conservative government's tougher policy towards Pyongyang.

Presidential spokesman Lee Dong-Kwan described the missile launches as part of a regular military exercise. "I believe North Korea will not sour relations with South Korea," he said.

But one analyst said it was "highly possible" the situation would worsen.

"We may see naval clashes (in disputed waters) in the Yellow Sea," said Yang Moo-Jin of the University of North Korean Studies, adding that the North seeks to sway the outcome of the April 9 parliamentary election.

Yonhap news agency said three or four missiles were fired into the Yellow Sea. It said they were Russian-designed Styx ship-to-ship missiles with a range of 46 kilometres (29 miles).

There were several similar launches last summer.

After a decade-long "sunshine" rapprochement policy under liberal presidents, the new Seoul administration is linking long-term economic aid to nuclear disarmament.

It says it will also raise Pyongyang's human rights record. On Thursday in Geneva, Seoul voted for a UN Human Rights Council resolution expressing deep concern at that record.

An international nuclear disarmament deal is currently deadlocked because of disputes between Washington and Pyongyang.

A North Korean foreign ministry spokesman warned that US delays in resolving the dispute could slow down work to disable its plutonium-producing atomic plants.

The spokesman, quoted by the official Korean Central News Agency, said the US was hindering progress in six-nation talks by raising "unjust demands."

North Korea last year agreed to disable its main atomic plants at Yongbyon and declare all its nuclear programmes and materials by the end of 2007.

The US-supervised disablement has been going ahead but Washington and Pyongyang are at odds over the declaration.

The North says it submitted the declaration last November. The United States says it has not fully accounted for a suspected uranium enrichment programme and for allegations of nuclear proliferation to Syria.

"If the US keeps insisting that what does not exist exists, and delays the settlement of the nuclear issue, it would have a serious impact on the disablement of nuclear facilities...," the North's spokesman said.

"We make it clear we have no uranium enrichment programme, we have not extended any nuclear help to any country. We have never dreamed of such things. There will never, ever be such things."

The North's decision Thursday to expel the 11 South Korean officials from the Kaesong complex, just north of the heavily fortified border, was in retaliation for comments by a South Korean minister.

Unification Minister Kim Ha-Joong said last week it would be difficult to expand the Seoul-funded complex unless the nuclear problem is resolved.

The Seoul presidential office said the expulsions could damage ties.

Kaesong is the most important joint project and most visible symbol of reconciliation between the two Koreas, who remain technically on a wartime footing. Almost 24,000 North Korean workers earning about 70 dollars a month produce light industrial goods there for 69 South Korean firms.

Newspapers urged the government to stand firm.

The largest-circulation newspaper Chosun Ilbo, in a comment headlined "We Must Weather the Storm," said Kim's comments simply reflect reality.

"The basic problem affecting the Korean peninsula is not the Kaesong Industrial Complex but North Korea's nuclear programme," Chosun said.

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