North Korea fired two suspected ballistic missiles into the sea Thursday, in its first substantive provocation to the new US administration of Joe Biden.

Thursday's launch comes after Pyongyang fired two short-range, non-ballistic missiles in a westerly direction towards China at the weekend, which US officials played down as not a violation of UN resolutions.

The nuclear-armed North has a long history of using weapons tests to ramp up tensions, in a carefully calibrated process to try to forward its objectives.

Donald Trump's first year in office was marked by a series of escalating launches, accompanied by a war of words between him and North Korean leader Kim Jong Un.

Pyongyang had been biding its time since the new administration took office, not even officially acknowledging its existence until last week.

But Seoul's joint chiefs of staff said the North fired two short-range missiles into the Sea of Japan, known as the East Sea in Korea, from South Hamgyong province.

They travelled 450 kilometres (280 miles) and reached a maximum altitude of 60 kilometres, the JCS added, and after an emergency meeting South Korea's National Security Council expressed "deep concern" at the launch.

North Korea is banned from developing any ballistic missiles under UN Security Council resolutions, and is under multiple international sanctions over its weapons programmes.

Japanese Prime Minister Yoshihide Suga was unequivocal, telling reporters: "North Korea launched two ballistic missiles."

It had been a year since the last such incident, he added, saying: "This threatens the peace and security of our country and the region. It is also a violation of the UN resolution." 

Biden warns of 'responses' if N.Korea 'choose to escalate'

President Joe Biden said the United States will "respond accordingly" if North Korea escalates its missile testing.

"We are consulting with our partners and allies," Biden said at his first White House press conference. "And there will be responses if they choose to escalate. We will respond accordingly."

Biden told reporters he was "prepared for some form of diplomacy" with North Korea but it "has to be conditioned upon the end result of denuclearisation." 

Pyongyang is banned from developing any ballistic missiles under UN Security Council resolutions, and is under multiple international sanctions over its weapons programs.

Biden said the North had violated UN resolutions by testing "those particular missiles."

The new US president also replied "yes" when asked if North Korea was the top foreign policy issue he was watching.