“We urge Europe to realise that the DPRK troops are now carrying an aggressive war in Europe against a sovereign European state,” he told a briefing after meeting Baerbock in Kyiv, using the acronym for North Korea’s official name.

Ukrainian president Volodymyr Zelensky, speaking later during his nightly video address, said a meeting of top commanders had considered a report by Ukrainian intelligence on the presence of North Korean troops in Russia. He repeated his call for greater action from Ukraine’s Western allies.

“There are already 11,000 in Kursk region,” Zelenskiy said, referring to the southern Russian region where Ukrainian troops have seized chunks of land since an incursion there in August.

“We see an increase in North Koreans, but we don’t see any increase in the reaction from our partners.”

US state department spokesperson Matthew Miller said Washington remained “greatly concerned about the partnership between North Korea and Russia”.

Miller said up to 10,000 North Korean troops have gone to Kursk, and could enter combat within days, up from an earlier US assessment of 8,000.

President Vladimir Putin has not expressly denied claims that North Korea had sent troops to Russia and said it was up to Moscow how to run its mutual-defence clause with Pyongyang.

Putin met North Korean foreign minister Choe Son-hui in the Kremlin on Monday, with Choe conveying “sincere, warm, comradely greetings” from North Korean leader Kim Jong-un.