S Korea, Japan worried as North fires off 4 missiles

North Korea fired four ballistic missiles into the sea off Japan's northwest coast early today, South Korean and Japanese officials said, days after the reclusive state promised retaliation over US-South Korea military drills.

South Korea's military said the missiles were unlikely to have been intercontinental ballistic missiles (ICBMs) which could reach the United States, but 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 to Japan's northwest 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,'' Japanese Prime Minister Shinzo Abe said in parliament, adding ''strong protests'' had been lodged with nuclear-armed North Korea.

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

The missiles were launched from the Tongchang-ri region near the 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 added.

''South Korea and the United States are conducting a close-up analysis, regarding further information,'' South Korea's Office of Joint Chiefs of Staff said in a statement.

The US military said it detected and tracked what it assessed was a North Korean missile launch, but it did not pose a threat to North America.

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 criticizes the annual drills, calling them preparation for war. It has previously conducted missiles launches timed to the joint military exercises.

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

North Korea test fired a new type of missile into the sea early last month, and has said it will continue to launch new strategic weapons.

Last month's test was the first since the election of US President Donald Trump, who has vowed to rein in North Korea and its young leader, Kim Jong Un.

Trump's national security deputies have reviewed in recent meetings a range of options to counter the North's missile threat, The New York Times reported. Options include direct missile strikes on the North's launch sites and the possibility of reintroducing nuclear weapons to the South, the Times said.

Those options will soon be presented to Trump and his top national security aides, the report said quoting US administration officials.

The United States withdrew nuclear weapons from South Korea in 1991 before the rival Koreas signed a declaration on denuclearisation of the Korean peninsula. North Korea has since walked away from the agreement, citing the threat of invasion by the United States.

The United States has about 28,500 troops and equipment stationed in the South, and plans to roll out the THAAD (Terminal High Altitude Area Defense) anti-missile defence system in the country by the end of the year.

Japan also plans to reinforce its ballistic missile defences and is considering buying either THAAD or building a ground-based version of the Aegis system that is currently deployed on ships in the Sea of Japan.