North Korea on Thursday fired a "long-range ballistic missile", South Korea's military said, Pyongyang's first weapons test since Seoul accused it of sending tens of thousands of soldiers to Russia.

South Korea's military had warned a day earlier that the nuclear-armed North was preparing to test another intercontinental ballistic missile (ICBM) or even conduct a nuclear test, possibly ahead of next week's US elections.

"Our military detected one ballistic missile launched from the Pyongyang area toward the East Sea at approximately 7:10 AM (2210 GMT) today," Seoul's Joint Chiefs of Staff said, referring to the body of water also known as the Sea of Japan.

"The ballistic missile is presumed to be a long-range missile launched at a high angle," it said.

North Korea typically test-fires its longest-range and most powerful missiles on a so-called lofted trajectory – fired up, not out – which it says is to avoid overflying neighbouring countries.

"Our military has heightened its alert level and is closely sharing information regarding North Korea's ballistic missile with the US and Japan authorities, maintaining a thorough readiness posture," Seoul's JCS added.

Tokyo also confirmed a possible launch, with Japan's coastguard reporting an "object potentially a ballistic missile launched from North Korea" and warning vessels to take care.

The launch came just hours after US and South Korean defence chiefs called on Pyongyang to withdraw its troops from Russia, where Washington says some 10,000 have been deployed for possible action against Ukrainian forces.

The defence chiefs of South Korea and the United States condemned North Korea's troop deployment to Russia in the "strongest" terms at the allies' annual defence talks at the Pentagon on Wednesday.

Seoul has previously said the nuclear-armed North's accelerating deployment posed a "significant security threat," with the military telling lawmakers on Wednesday that thousands of troops had been moved to western Russia.

"The fact that such a large number of personnel has moved westward suggests they are unlikely to have gone merely for observation," a presidential official said.

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.

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.

North Korea has denied sending troops, but in the first comment in state media last week, its vice foreign minister said that if such a deployment to happen, it would be in line with international law.

Pyongyang is banned from tests using ballistic technology by multiple rounds of UN sanctions, but Kim Jong-un has ramped up testing this year, with experts warning he could be testing weaponry before providing it to Russia. (AFP)