The tests came as Pyongyang warned that US demands over its nuclear programme could further delay its disabling of nuclear facilities as agreed last year.

A South Korean presidential spokesman said the missile tests seemed to be part of "ordinary military training".

North Korea's actions come a day after it expelled South Korean managers from a joint industrial park on the border.

The expulsions are being seen as a diplomatic protest against the Seoul government which has said it would link progress at the park with progress on the North's denuclearisation effort, says the BBC's Kevin Kim in the South Korean capital.

The conservative government of newly-elected President Lee Myung-bak has made it clear that further economic co-operation with Pyongyang will only be possible if North Korea fully disables its nuclear facilities.

US nuclear suspicions

A government official told South Korea's Yonhap news agency that North Korea launched several missiles at 1030 local time (0130 GMT).

The official said that the missiles were fired into the Yellow Sea.

In response, a South Korean presidential spokesman told Yonhap that "the government regards North Korea's missile firing as merely a part of its ordinary military training".

Analysts say such launches are often timed to make a political point.

Meanwhile, a North Korean foreign ministry spokesman warned the US not to pursue its suspicions that Pyongyang has hidden some of its nuclear programme.

Under a landmark disarmament-for-aid deal reached last year, North Korea has taken steps to shut down its main atomic facility at Yongbyon.

But the US wants North Korea to fully declare its nuclear weapons stockpile and answer US suspicions of nuclear proliferation and possession of a secret uranium enrichment programme.