North Korea fired four ballistic missiles into the sea off Japan's northwest early on Monday, South Korean and Japanese officials said, days after it 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 can reach the United States. The missiles flew on average 1,000 km and reached a height of 260 km.

Some of the missiles landed in waters as close as 300 km from Japan's northwest coast, Japan's Defence Minister Tomomi Inada said in Tokyo.

Japanese Prime Minister Shinzo Abe said "strong protests" had been lodged with nuclear-armed North Korea.

"The launches are clearly in violation of (United Nations) Security Council resolutions. It is an extremely dangerous action," Abe told parliament.

South Korea's acting President Hwang Kyo-ahn condemned the launches as a direct challenge to the international community and said Seoul would swiftly deploy a US anti-missile defence system despite angry objections from China.

The missiles were launched from the Tongchang-ri region near the reclusive North's border with China, South Korean military spokesman Roh Jae-cheon told a briefing.

It was too early to say what the relatively low altitude indicated about the types of missiles, he said.

Joint drills

North Korea had threatened to take "strong retaliatory measures" after South Korea and the United States began annual joint military drills on Wednesday that test their defensive readiness against possible aggression from the North.

North Korea criticises the annual drills and has previously conducted missile launches to coincide with the exercises.

Last year, North Korea fired a long-range rocket from Tongchang-ri that put an object into orbit. That launch was condemned by the United Nations for violating resolutions that ban the use of ballistic missile technology.

US condemns North Korea's launch of missiles

The United States condemned North Korea's launch of four ballistic missiles, vowing that Washington was ready to “use the full range of capabilities at our disposal against this growing threat.”

“The United States strongly condemns the DPRK's ballistic missile launches tonight, which violate UN Security Council Resolutions explicitly prohibiting North Korea's launches using ballistic missile technology,” the State Department's acting spokesman Mark Toner said in a statement.

Toner added: “We remain prepared — and will continue to take steps to increase our readiness — to defend ourselves and our allies from attack, and are prepared to use the full range of capabilities at our disposal against this growing threat.”

The US State Department said Pyongyang's bellicose actions will only help strengthen the already “ironclad” resolve of Washington and its allies against North Korea's “unlawful conduct.”

“We call on all states to use every available channel and means of influence to make clear to the DPRK and its enablers that further provocations are unacceptable, and take steps to show there are consequences to the DPRK's unlawful conduct,” Toner added.

“Our commitment to the defence of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad.“