North Korean troops have been killed during combat against Ukrainian forces, the first reported casualties since North Korea sent troops to support Russia, according to US and Ukrainian authorities.

About 30 North Korean troops were killed or wounded during a battle with the Ukrainian army over the weekend in Russia's Kursk border region, according to Ukraine's military intelligence agency.

North Korea has so far sent 10,000 to 12,000 troops to Russia to help it in the almost-three-year war.

Meanwhile, Russia's defence minister Andrei Belousov said Moscow must be ready to fight the NATO military alliance in Europe in the next decade.

"The activities of the Ministry of Defence are based on … ensuring full readiness for any scenario in the medium term, including a possible military conflict with NATO in Europe in the next decade," Mr Belousov told an audience including President Vladimir Putin at his ministry.

Mr Belousov pointed to US plans to modernise its nuclear forces, the opening of a US missile defence base in Poland, new NATO combat readiness plans, and plans announced at the July NATO summit to deploy US medium-range missiles in Germany in 2026, as reasons for Russia's preparedness.

North Koreans on front lines: US

The casualties of the North Korean soldiers occurred around three villages in Kursk, where Russia has for four months been trying to quash a Ukrainian incursion, the agency, known by its acronym GUR, said in a public post on the Telegram messaging app.

At least three North Korean servicemen went missing around another Kursk village, GUR said.

The White House also said it believed North Korean troops were on the "front lines" of Russia's war and were "actively engaged in combat operations" against Ukraine.

US national security spokesman John Kirby confirmed the assessment after Ukraine's government said North Korean troops had moved from support roles into direct fighting on behalf of Russia.

Mr Kirby said North Korean troops were taking casualties in the fighting and promised a strong US and allied sanctions response to North Korea.

Pentagon press secretary Major General Pat Ryder told reporters that some North Korean troops had died in combat in Kursk but did not have a specific number of those killed or wounded.

Those troops have primarily been used in an infantry role and started fighting in combat operations about a week ago, Mr Ryder said.

Kremlin spokesman Dmitry Peskov referred questions to the Russian Defense Ministry, which didn't immediately comment.

North Korean leader Kim Jong Un has pledged unwavering support for Russia's full-scale invasion of its neighbour under a mutual defence pact.

The alliance gave a jolt to international relations and Mr Putin said the planned deployment of US intermediate-range missiles to Europe and Asia had brought new threats.

"In view of rising geopolitical tensions, we must take additional measures to ensure the security of Russia and our allies," Mr Putin told a meeting with top military brass.

"We are doing it accurately and in a balanced way to avoid being drawn into a full-scale arms race."

Language barrier hampers combat coordination

However, military analysts said the language barrier had bedevilled combat coordination between Russian and North Korean troops.

"The poor integration and ongoing communication problems between Russian and North Korean forces will likely continue to cause friction in Russian military operations in Kursk … in the near term," the Institute for the Study of War, a Washington think tank, said.

In November, Ukrainian officials said their forces had for the first time engaged with North Korean units that had been recently deployed to help Russia.

Ukraine seized land in Russia's Kursk border region last August in what was the first occupation of Russian territory since World War II.

The operation embarrassed the Kremlin and aimed to counter unceasingly glum news from the front line.

The incursion hasn't significantly changed the war's dynamics.

Over the past year, Russia has been on the front foot, with the exception of Kursk, and has been grinding deeper into eastern Ukraine's Donetsk region despite heavy losses.

Mr Putin said large numbers volunteering for service were turning the tide of the Ukraine war in Moscow's favour, as open-source maps suggested his army was advancing at the fastest pace since 2022.

The Russian Defence Minister also said the military had been making steady gains in Ukraine, claiming it had accelerated recently, with Russian forces capturing about 30 square kilometres of territory a day.

AP/Reuters