North Korea has fired four ballistic missiles into waters off its east coast, Japanese Prime Minister Shinzo Abe says, adding that Toyko has lodged a “stern protest” with Pyongyang.

The launch of the missiles, three of which Abe said fell into Japan’s exclusive economic zone, were “a grave threat to our country’s national security,” according to government spokesman Yoshihide Suga.

South Korean news agency Yonhap, which first reported the launch, said an “unidentified projectile” had been launched from an area near the Dongchang Ri missile site at 7.36 am local time and flew across North Korea before landing in the East Sea, also known as the Sea of Japan.

The test was an apparent protest against ongoing military drills between South Korea and the United States, Yonhap said.

A South Korean military official told Yonhap the projectile could be an intercontinental ballistic missile (ICBM) capable of reaching the US mainland.

North Korea said in February it had successfully test-fired a medium-range ballistic missile – not an ICBM – capable of carrying a nuclear warhead. It was the first such test and provocation since US President Donald Trump took office.

In his New Year’s address, North Korean leader Kim Jong Un had said the reclusive state was almost ready to test-fire an intercontinental ballistic missile.

Last month, US Defence Secretary James Mattis warned North Korea any nuclear attack on the United States or its allies would be met with an “effective and overwhelming” response.

Pyongyang has ramped up its nuclear programme under Kim, carrying out two nuclear tests and launching some 20 ballistic missiles last year alone.

The missiles landed in waters as close as 300 km to Japan’s northeast coast, Japan’s Minister of Defence said later.

Three of the four missiles landed within Japan’s exclusive economic zone, 300-350 km from the Oga Peninsula in Akita prefecture, Tomomi Inada said in Tokyo.

South Korea’s acting president Hwang Kyo-ahn said Seoul should swiftly complete the deployment of a US anti-missile defence system after North Korea’s action.

“We should quickly finish the deployment of THAAD (Terminal High Altitude Area Defense) deployment and acquire a defence system against North Korea’s nuclear missiles,” said Hwang in opening remarks while presiding over a National Security Council meeting after the North’s latest provocation.