Ukrainian troops began their incursion into the Kursk region in August and still control some settlements there, part of Kyiv's attempt to relieve pressure on its forces in eastern Ukraine, where Russia has been making steady advances.
"Today there are already preliminary data that the Russians have begun using soldiers from North Korea in the assaults - a noticeable number," Zelenskiy said in his regular address to the nation.
"The Russians include them in consolidated units and use them in operations in the Kursk region. For now, it is only there."
Zelenskiy said the North Korean troops' participation in the operations in the Kursk region, which borders Ukraine, amounted to a new escalation in the nearly 34-month-old war.
Ukraine would continue to defend itself, including against the North Korean troops, Zelenskiy said.
Ukrainian and South Korean officials have previously said that more than 10,000 North Korean troops were in Russia.
Zelenskiy said Russian President Vladimir Putin was taking steps to expand and continue the war.
He issued a fresh appeal to Ukraine's Western allies to strengthen their support for Kyiv, saying that he would discuss it with European partners next week.
Zelenskiy plans to attend a meeting with the leaders of Britain, France, Germany, Italy, Poland, NATO, and the EU in Brussels on Wednesday.
Meanwhile, Ukraine's military leadership has replaced the commander overseeing defences in the eastern Donetsk region where Russian forces are making swift advances, a military official says.
General Oleksandr Tarnavskiy, 54, was appointed to head the operational and tactical group Donetsk, replacing General Oleksandr Lutsenko, the official at the General Staff told Reuters.
Lutsenko was criticised by Ukrainian military bloggers and some lawmakers for failing to stop Russian troops' relentless push toward the strategic Ukrainian city of Pokrovsk.
The battles around Pokrovsk, a key logistic hub for Ukrainian military and civilians in the eastern Donetsk region, come at a critical juncture of the war.
Ukraine is on the back foot on the battlefield as Russian troops advance at their fastest pace since the early days of Moscow's invasion in February 2022.
Today, Russia attempted to overload our air defense systems, as they always do during such massive attacks. This time, they deliberately waited for freezing weather to strike, aiming to make life even harder for people. The attack coincided with a cold snap—a deliberate, cynical… — Volodymyr Zelenskyy / Володимир Зеленський (@ZelenskyyUa) pic.twitter.com/Clb2ZEDYyIDecember 13, 2024
Last month, Zelenskiy replaced several other generals, saying changes were needed.
Kyiv is also trying to build ties with US President-elect Donald Trump, whose promise to end the war swiftly has raised concerns in Ukraine that it could be largely on Moscow's terms.
Ukraine's Khortytsiya military command said on Saturday that Russian troops had destroyed several Ukrainian positions in villages around Pokrovsk, improving their tactical positions after "exhausting battles" and forcing Kyiv forces to retreat.
In its daily report, the Ukrainian military reported a total of 62 combat clashes near Pokrovsk in the last 24 hours.
Ukrainian officials said that about 11,000 people still remain in Pokrovsk, which has been under constant shelling for months and where all critical infrastructure - electricity, water, and gas supplies were destroyed.
Pokrovsk lies at the intersection of several important roads and rail lines and is just about 20 kilometres away from the administrative border of the Donetsk region.
Moscow has been trying to reach that line since the start of the invasion, as its forces aim to seize all of Donetsk and Luhansk regions.
Military analysts said that for Kyiv, losing Pokrovsk would also potentially mean having the war closer to central Ukraine and its better-protected rear.
