"North Korea fires unidentified ballistic missile," Seoul's Joint Chiefs of Staff said, without giving further details, adding that an analysis was under way.

It said the missile was fired off North Korea's east coast, into what South Korea calls the East Sea, also known as the Sea of Japan.

Tokyo also confirmed the launch, with the prime minister's office saying Pyongyang had "launched a suspected ballistic missile".

On Thursday, the nuclear-armed North test-fired what it said was its most advanced and powerful solid-fuel intercontinental ballistic missile (ICBM).

That launch was Kim Jong Un's first weapons test since being accused of sending soldiers to Russia.

It also came just hours after US and South Korean defence chiefs called on Pyongyang to withdraw its troops, warning that North Korean soldiers in Russian uniforms were being deployed for possible action against Ukraine.

On Sunday, South Korea, Japan, and the United States conducted a joint air drill involving a heavy bomber in response to the ICBM launch.

The drill mobilised the US' B-1B bomber, South Korea's F-15K and KF-16 fighter jets, and Japan's F-2 jets.

Such joint drills infuriate Pyongyang, which views them as rehearsals for invasion.

- 'Aggressive nature' -

Kim Yo Jong, sister of the country's leader and a key spokesperson, said the drill was "just another outright action-based explanation of the most hostile and dangerous aggressive nature of the enemy toward our Republic."

In a statement carried Tuesday by the official Korean Central News Agency, she said the drill was "absolute proof of the validity and urgency of the line of building up the nuclear forces we have opted for and put into practice."

She warned that any "upset of the balance of power between rivals on the Korean peninsula and in the region precisely means a war."

Experts have suggested the spate of weapons tests by Pyongyang could be a bid to distract attention from its purported troop deployment to Russia -- or bump itself up the agenda ahead of the US election.

Seoul has long accused the nuclear-armed North of sending weapons to help Moscow fight Kyiv and alleged that Pyongyang has moved to deploy soldiers en mass in the wake of Kim Jong Un's signing of a mutual defence deal with Russian President Vladimir Putin in June.

It has also warned that Russia may be providing new technology or expertise in return for weapons and troops to help them fight Ukraine.

Seoul, a major weapons exporter, has said it is reviewing whether to send weapons directly to Ukraine in response, something it has previously resisted due to longstanding domestic policy that prevents it from providing weaponry into active conflicts.

North Korea has denied sending troops, but its vice foreign minister has said any such deployment would be in line with international law.