North Korean leader Kim Jong Un supervised the test launches of four missiles by an army unit commissioned with attacking US military bases in Japan, the country's official news agency reports.

North Korea fired four ballistic missiles into the sea off Japan's northwest yesterday, angering South Korea and Japan, days after it promised retaliation over US-South Korea military drills it sees as preparation for war.

"Involved in the drill were Hwasong artillery units of the KPA Strategic Force tasked to strike the bases of the US imperialist aggressor forces in Japan in contingency," the North's official KCNA news agency said.

"In the hearts of artillerymen ... there was burning desire to mercilessly retaliate against the warmongers going ahead with their joint war exercises," KCNA said.


"He (Kim) ordered the KPA Strategic Force to keep highly alert as required by the grim situation in which an actual war may break out any time, and get fully ready to promptly move, take positions and strike so that it can open fire to annihilate the enemies."

Japanese Prime Minister Shinzo Abe and US President Donald Trump discussed North Korea's missile launches during a phone call today, the Jiji and Kyodo news agencies reported.

It came as the Secretary-General of the United Nations condemned North Korea's action.

"Such actions violate Security Council resolutions and seriously undermine regional peace and stability," a spokesman for UN chief Antonio Guterres said in a statement.

Guterres also called on North Korean leaders once again to refrain from further provocations and return to full compliance with the country's international obligations.

- Reuters, AAP