The last time North Korea tested this many weapons in a month was in 2019, after high-profile negotiations between leader Kim Jong Un and then-US president Donald Trump collapsed.

Since then, talks with the US have stalled, and the country is reeling economically from biting international sanctions and a self-imposed coronavirus blockade.

"North Korea fired an unidentified projectile towards the East Sea (Sea of Japan)," Seoul's Joint Chiefs of Staff said in a statement.

Japan's coast guard meanwhile said it had detected a launch of a "possible ballistic missile".

This year Pyongyang has embarked on a fresh flurry of sanctions-busting tests, including hypersonic missiles, after Kim re-avowed his commitment to military modernisation at a key party speech in December.

Pyongyang carried out two weapons tests last week, and has conducted at least four additional tests this month -- including of what it called hypersonic missiles on January 5 and 11.

Friday saw state media release photos showing Kim wearing his usual long black belted leather jacket, surrounded by uniformed officials -- their faces pixelated out -- inspecting a munitions factory that produces "a major weapon system".

The North has not tested intercontinental ballistic missiles or nukes since 2017, putting launches on hold as Kim embarked on a blitz of high-level diplomacy through three meetings with then-US president Donald Trump and other top leaders.

But this month the regime hinted it could restart such launches, blaming US "hostile" policy for forcing its hand.

The string of launches in 2022 comes at a delicate time in the region, with Kim's sole major ally China set to host the Winter Olympics next month and South Korea gearing up for a presidential election in March.

Domestically, North Korea is preparing to celebrate the 80th anniversary of the birth of late leader Kim Jong Il in February, as well as the 110th birthday of founder Kim Il Sung in April.

With reports of soaring food prices and worsening hunger, an economically-reeling Pyongyang recently restarted cross-border trade with neighbouring China.

And ally Beijing, along with Russia, this month blocked the UN Security Council from imposing fresh sanctions in response to the recent tests.