Zelensky had revealed the transfer at a speech at the UN General Assembly in September, adding that the two North Koreans had not been "easy to capture alive".

It prompted a swift condemnation from South Korean President Lee Jae Myung, who called it "greatly disappointing" and said the transfer was a "highly sensitive issue" impacting national security.

President Lee also faced heavy criticism at home in light of the revelation, with local opposition accusing him of wanting to cover up the transfer to appease Pyongyang.

But his office said that it had been kept confidential partly to protect the North Koreans' families and added that Kyiv also wanted the transfer kept secret to avoid criticism that it had given up an opportunity to exchange the soldiers for Ukrainian prisoners of war.

Lee's office explained Ukraine had indicated the soldiers would be mentioned in Zelensky's address, but not the fact of their transfer.

The spat escalated when Zelensky's office responded claiming that the two countries did not have a non-disclosure agreement.

Seoul later demanded Ukraine apologise for "announcing a falsehood" and said its actions had seriously damaged trust between the two nations.

Seoul has been sympathetic to Ukraine since Russia's full-scale invasion, but has refused to break from its longstanding policy of not providing lethal weapons to any country engaged in active war.

After North Korean troops were deployed to support Russia in 2024, Kyiv stepped up pressure on South Korea to supply cutting-edge anti-air missiles to help defend Ukraine's energy infrastructure from Russian drone and missile attacks.

This July, Seoul had announced a $100 million assistance package to Ukraine. Lee's office said it would be an expansion of earlier assistance which was comprised of non-lethal items including helmets, personal armour and medical equipment.

Meanwhile Russia's foreign ministry has warned it would have "no choice but to retaliate" if South Korea provided Ukraine with arms, without specifying action it might take.

South Korea has long pursued cordial ties with its neighbours who may be at odds with its ally, the US.

Since the collapse of the Soviet Union, it has worked to deepen trade and diplomatic relations with Russia, which borders North Korea, and views both Moscow and Beijing as key partners in efforts to influence Pyongyang.