Ukrainian President Volodymyr Zelenskyy has expressed concern that fighting between Ukrainian forces and North Korean troops will not only bring instability to the war situation but also to the international community.

In a video address on Tuesday, the president said, "The first battles with North Korean soldiers open a new page of instability in the world."

Zelenskyy also said, "Together with the world, we must do everything to make this Russian step to expand the war -- to really escalate it -- to make this step a failure."

But he stopped short of providing details on Ukraine's engagements with North Korean troops.

The New York Times on Tuesday reported that according to a senior Ukrainian and a senior US official, North Korean troops have entered the fight in Russia's war against Ukraine. They clashed for the first time with Ukrainian forces who are occupying a large chunk of Russia's Kursk region.

Ukraine's military has been conducting cross-border attacks in the Kursk region since August.

It was unclear when the fighting took place. The Ukrainian official said the engagement was limited, and likely meant to probe the Ukrainian lines for weaknesses.

The newspaper said the Ukrainian official offered no details about casualties, but the US official said a significant number of North Korean troops were killed.