President Putin met North Korea's foreign minister in the Kremlin on Monday amid mounting concern in the West that North Korean soldiers are about to enter the Ukraine war on Moscow's side.

Video of the meeting showed the pair shaking hands for a full minute as Putin greeted Choe Son Hui.

Putin noted that they were meeting on Russia's National Unity Day, a public holiday, and Choe conveyed "sincere, warm, comradely greetings" from North Korean leader Kim Jong Un.

The United States says North Korea has sent some 10,000 troops to Russia, including 8,000 to the western Kursk region where Russia is still battling to expel Ukrainian troops who broke across the border in August.

Putin, who signed a treaty with Kim in June that includes a mutual defence clause, has neither confirmed nor denied the North Korean troop presence. He has said that Russia is free to implement the pact as it sees fit.

The Russian leader briefly alluded to the military situation in Kursk when he met youth volunteers earlier on Monday, telling them: "When the enemy is cleaned out from the region, there will be lots of work for you to do."

In Kyiv, Ukrainian Foreign Minister Andrii Sybiha said he had discussed with his German counterpart Annalena Baerbock the "need for decisive action" in response to North Korean involvement in the war in Ukraine.

He said North Korean troops were waging "an aggressive war in Europe against a sovereign European state".

Ukrainian President Volodymyr Zelenskiy last week urged Kyiv's allies to stop just "watching" and start taking action to tackle the presence of the North Koreans. (Reuters)