A North Korean missile launch is being investigated as a possible test of a submarine-launched ballistic missile, according to South Korean officials.
The secret state fired at least one projectile toward the eastern sea in what appeared to be a demonstration of better military capabilities.
If North Korea has developed a missile that can be launched from a submarine, it means that weapons can travel beyond their current range.
The US is due to restart nuclear talks with Pyongyang this week after talks collapsed in February.
North Korea is understood to be willing to offer partial denuclearization in exchange for the US suspending disabling sanctions and providing security guarantees.
But on Wednesday morning, Japanese cabinet chief secretary Yoshihide Suga said the north fired two ballistic missiles within minutes of one another, one of which appeared to have landed in Japan's exclusive economic zone off the northwest coast. .
There were no reports of damage to boats or aircraft in the area.
It is the first time since November 2017 – when Northern nuclear and missile launches have been particularly provocative – that a weapon has been launched in the EEZ.
The South Korean military reported launching a missile, which it said flew 280 miles.
A photo from North Korea's Central News Agency of an August missile test
In response to the releases, Japanese Prime Minister Shinzo Abe said he violated UN resolutions against North Korea.
"We will continue to cooperate with the US and the international community and will do our utmost to maintain and protect people's safety as long as we remain alert," he said.
The Seoul National Security Council also expressed "strong concern" about the test.
The launches are North Korea's ninth weapon testing round since late July.
Kim Dong-yung, an analyst with the Seoul Far East Institute of Studies and a former military officer who took part in inter-Korean military talks, said the test was an attempt to gain strength before negotiations with the US curriculum.
The two countries will contact each other on Friday before conducting negotiations on Saturday.
North Korea's first deputy foreign minister, Choe Son Hui, said in a statement: "It is my expectation that labor-level negotiations will accelerate the positive development of relations between North Korea and the US."
President Donald Trump and Kim Jong Un Shake Hands
However, the state has said it will never unilaterally surrender its nuclear weapons and missiles and wants sanctions against them lifted before any progress can be made in the negotiations.
Choe Son Hui had already warned that negotiations could be final if they stopped again.
But some positive noises were made after North Korea praised President Trump for suggesting that the US could follow a "new method" not specified in the talks.