North Korea may have fired an intercontinental ballistic missile capable of reaching the US mainland, South Korea's military said after it spotted several unidentified projectiles landing in the sea between Japan and the Korean Peninsula.

In Japan, the Government said that it had detected four missiles coming from North Korea and that three had landed perilously close to Japan, splashing down within its exclusive economic zone.

"These missile launches clearly show that North Korea has developed a new threat," a visibly worried Prime Minister Shinzo Abe told reporters in Tokyo. "We will collect information and strongly protest to North Korea."

The launch coincides with annual exercises between the United States and South Korean militaries and is considered another display of Pyongyang's anger about the exercises, which it views as a pretext for an invasion.

The apparent missile was launched from a known long-range missile site on the west coast, not far from the border with China.

"We are conducting an analysis on the projectile to determine its type and flight range. It will take a while before we come up with a final analysis," the joint chiefs said, according to Yonhap News Agency.It flew more than 1000km across the country before splashing into the Sea of Japan, Seoul's Joint Chiefs of Staff said in a text message to local reporters.

South Korea's national security council convened an emergency meeting to discuss the launch.

North Korea has repeatedly claimed to be working on an intercontinental ballistic missile capable of reaching the West Coast of the United States, and has been making observable progress toward this goal. In his latest New Year's address, Kim Jong Un said North Korea had "entered the final stage of preparation for the test launch an intercontinental ballistic missile".

Regardless of whether the launch turned out to be an ICBM, it is just a matter of time until North Korea succeeds in its goal, said Jeffrey Lewis, director of the East Asia Non-Proliferation Programme at the Middlebury Institute of International Studies at Monterey in California.

"It doesn't matter if it's today or tomorrow or next week or next year - that's where this is heading," Lewis said.

"But we have no plan other than saying this is unacceptable or that it won't happen," he said, referring to a tweet from President Donald Trump earlier this year.

After Kim's statement in his January 1 address that North Korea was working on its ICBM program, Trump tweeted: "It won't happen!"

His Administration is reviewing policy towards North Korea, which was characterised as "strategic patience" during the Obama Administration - waiting for sanctions to hurt and a humbled Kim to come to the negotiating table.

The latest provocation came as large-scale military exercises, involving more than 320,000 South Korean and US troops and high-tech US firepower, continue on the southern half of the peninsula. They began last week and will continue through the end of April.

In the past year or two, the exercises have become more overtly offensive, with the two militaries practicing "decapitation strikes" on the North Korean leadership.

North Korea denounced the exercises and warned last week that it was ready to retaliate. North Korea "will never remain a passive onlooker to the new US Administration overtly revealing its intention to put military pressure on [North Korea] and invade it while crying out for 'peace by dint of strength,' " the state-run Korean Central News Agency reported in a statement it attributed to the Foreign Ministry.

North Korea last month launched an intermediate-range missile, its first since Trump was elected president. The missile appeared to show significant technological advances, with upgraded power and range, and analysts said it could mark another step in the push toward the capacity to hit Alaska or Washington state.

After that, Kim's regime is suspected of ordering the assassination on the leader's half brother, Kim Jong Nam, who was attacked with a chemical weapon at the airport in Kuala Lumpur, Malaysia, and died within 20 minutes.

The assassination led the Trump Administration to cancel visas for North Korean diplomats to go to New York for meetings with former US officials involved in North Korea policy, which would have been the first time in more than five years that such a meeting had taken place on US soil.