North Korea fired four ballistic missiles into the sea off Japan's north-west coast early on Monday, South Korean and Japanese officials said, days after the reclusive state promised retaliation over US-South Korea military drills it sees as a preparation for war.

South Korea's military said the missiles were unlikely to have been intercontinental ballistic missiles (ICBM) which could reach the United States, but flew on average 1,000km (600 miles) and reached a height of 260 km (160 miles).

Some of the missiles landed in waters as close as 300km (190 miles) to Japan's north-west coast, Japan's defence minister Tomomi Inada said in Tokyo.

"The launches are clearly in violation of (UN) Security Council resolutions. It is an extremely dangerous action," Shinzo Abe, the Japanese prime minister, said in parliament, adding that "strong protests" had been lodged with nuclear-armed North Korea.

Hwang Kyo-ahn, South Korea's acting president, condemned the launches as a direct challenge to the international community and said the country would swiftly deploy a US anti-missile defence in the face of angry objections from China.