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

It was not immediately clear what type of missile was fired; Pyongyang has staged a series of missile test-launches of various ranges in recent months. The ramped-up tests come as leader Kim Jong-un pushes for a nuclear and missile program that can deter what he calls U.S. and South Korean hostility toward the North.

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 1,000 kilometres 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 U.S. anti-missile defence 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.

NORTHKOREA-MISSILES/SHINZO ABE

Japan's Prime Minister Shinzo Abe, seen at an upper house parliamentary session in Tokyo on Monday after reports on North Korea's missile launches, called them 'extremely dangerous' acts in violation of international law. (Issei Kato/Reuters)

"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 U.S. 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.

Chinese Foreign Ministry spokesman Geng Shuang said at a daily news briefing that China, which is holding its annual meeting of the National People's Congress, had noted North Korea's latest action.

"All sides should exercise restraint and not do anything to irritate each other to worsen regional tensions," Geng said, referring to both the missile launch and U.S.-South Korean military exercises.

International test for Trump

Russia also expressed its condemnation.

"Definitely, we are seriously worried – these are the sort of actions that lead to a rise in tension in the region and of
course in this situation, traditionally, Moscow calls for restraint from all sides," Kremlin spokesperson Dmitry Peskov told a conference call with
reporters.

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 U.S. President Donald Trump, who has vowed to rein in North Korea and its young leader, Kim Jong-un.

North Korea Rockets Photo Gallery

A soldier stands at a launching site in Tongchang-ri, North Korea, in April 2012. South Korea said Monday's launches were made from the Tongchang-ri area, where it has conducted rocket launches in recent years. (David Guttenfelder/Associated Press)

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 U.S. administration officials.

Nukes in South 'total nonsense': politician

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

"The claim that we should redeploy nuclear weapons here, 20 years after they were withdrawn, is total nonsense," said Woo Sang-ho, floor leader of South Korea's main opposition Democratic Party. "I am formally asking the United States not to bring this issue up for consideration," Woo said in a party meeting.

North Korea conducted its fifth and most powerful nuclear test last September, following what the United States said was an "unprecedented" level of activity in its banned nuclear and missile programmes.

State media said after that test Pyongyang had used a nuclear warhead small enough to mount on a ballistic missile.

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.