Nato and South Korea have expressed alarm that North Korean troops could soon be joining Moscow's side in the Ukraine-Russia war.

Nato said on Monday thousands of North Korean troops were moving toward the front line, a development which has prompted Kyiv to call for more weapons and an international plan to keep those troops at bay.

Western leaders said North Korea has sent some 10,000 soldiers – up from an estimate of 3,000 troops last Wednesday – to help Russia’s military campaign, and warned that its involvement in a European war could also unsettle relations in the Indo-Pacific region, including Japan and Australia.

For his part, Ukraine's President Volodymyr Zelensky said he had talked with South Korean President Yoon Suk-yeol and told him that 3,000 North Korean soldiers are already at military bases close to the Ukranian front line, and that he expects that deployment to increase to 12,000.

"This war is becoming internationalised, extending beyond two countries," Zelenskiy said on X.

"We agreed to strengthen intelligence and expertise exchange, intensify contacts at all levels, especially the highest, in order to develop an action strategy and countermeasures to address this escalation," Zelensky said.

Yoon told Zelensky that if North Korea receives aid from Russia and is able to glean military experience and knowledge from its involvement in the war, it would pose a "great threat" to South Korea's security, his office said.

South Korea has said it may start supplying weapons to Ukraine if North Korean troops joined Russia's war. Neither Putin nor Pyongyang has denied North Korean troops are in Russia.

Meanwhile, Moscow said on Tuesday it had seized the war-battered mining town of Selydove in eastern Ukraine where Kyiv's forces are buckling.

Moscow also said it had wrested control of the nearby villages of Bogoyavlenka, Girnyk and Katerynivka, also in the Donetsk region, which Russian President Vladimir Putin claimed was formally part of Russia in late 2022, the year Moscow started the war.

The gains announced by Moscow on Tuesday are just the latest in a string of Russian advances that have gained momentum since February with the collapse of Ukraine's defences in the stronghold town of Avdiivka.

At the same time, North Korean Foreign Minister Choe Son-hui arrived in Russia's far east on Tuesday on her way to Moscow, Russian state media said. Russian state news agencies said it was not clear who Choe, making her second visit in six weeks, would meet.

The Kremlin said Russian President Vladimir Putin had no plans to meet her. (Agencies)