Kyiv, Ukraine - Ukrainian President Volodymyr Zelensky said Friday that North Korean troops were back on the front line in Russia's Kursk region, after reports Moscow had withdrawn them due to heavy losses.

More than 10,000 soldiers from the reclusive state were sent to Russia last year to help it fight back a shock Ukrainian offensive into the border region, according to South Korean and Western intelligence.

A Ukrainian military spokesman told AFP last Friday that Kyiv had not encountered activity or clashes with North Korean troops for three weeks.

"There have been new assaults in the Kursk operation areas... the Russian army and North Korean soldiers have been brought in again," Zelensky said in his evening address.

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The Ukrainian leader said a "significant number" of opposing troops had been "destroyed".

"We are talking about hundreds of Russian and North Korean soldiers," he added.

Kyiv captured dozens of border settlements in its Kursk assault six months ago, the first time a foreign army had crossed into Russian territory since World War II.

The North Korean deployment, never officially confirmed by Moscow or Pyongyang, was supposed to reinforce the Russian army and help them expel Ukraine's troops.

But as of February Ukraine still holds swathes of Russian territory, something Zelensky sees as a bargaining chip in any future negotiations with Moscow.