North Korea on Monday fired four ballistic missiles towards the Sea of Japan, with three of them falling into Japan’s exclusive economic zone, Japan Prime Minister Shinzo Abe said, according to Reuters. The South Korean military claimd the missiles, which flew for 1,000 km, were fired from Tongchang-ri region near the China border.

“The launches are clearly in violation of [United Nations] Security Council resolutions. It is an extremely dangerous action,” Abe said in parliament. He added that North Korea had “entered a new stage of threat”.

It was unclear what type of missiles were fired, BBC reported, adding the the South Korean military was investigating to determine the type of projectile used.

South Korea and the United States had started their annual joint military drills on Wednesday. North Korea had then threatened to take “strong retaliatory measures”, Reuters added.

On February 13, North Korea had test-fired the Pukguksong-2 missile test, overseen by their leader Kim Jong-un. The missile, which can carry nuclear warheads, had reportedly used a solid-fuel engine, which gives ballistic rockets more power and range. The US, Japan and South Korea have reached out to the United Nations Security Council to protest against the missile test.

Earlier in February, the UN had warned North Korea against “provocative actions”. Kim Jong-un had claimed it was close to test-launching an intercontinental ballistic missile chi was capable of reaching American shores.