Volodymyr Zelenskyy has called on the UK and other Ukrainian allies to stop "watching" and provide long-range weapons to strike North Korean troops in Russia before they enter combat.
The "first thousands of soldiers from North Korea are near the Ukrainian border" and Ukrainians "will be forced to defend themselves against them", the Ukrainian president said in a Telegram video.
"And the world will watch again."
Mr Zelenskyy said Ukraine had pinpointed every location where North Korean soldiers had been stationed in Russia, but said Kyiv's allies had not supplied the long-range weapons needed to hit them.
"But instead of such necessary long-range capability, America watches, Britain watches, Germany watches...," he said.
"Everyone in the world who truly wants the Russian war against Ukraine not to expand... must not just watch. They must
act. Words about the inadmissibility of escalation and expansion of war must be matched with actions."
The video showed images of North Korean soldiers and missile launches as well as images of the war in Ukraine.
Mr Zelenskyy previously criticised what he said was a "nothing" response from his allies about Russia deploying North Korean troops for the war in Ukraine.
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'Growing alliance of aggression'
Meanwhile the UK defence secretary warned of a "growing alliance of aggression" between North Korea and Russia.
John Healey said UK Defence Intelligence had confirmed 10,000 North Korean soldiers had arrived in Russia and a "significant proportion" were heading for the Kursk region, where Ukrainian forces launched an invasion in August.
The Ministry of Defence (MoD) said it was "almost certain some have already deployed to the Kursk region" in its latest intelligence update.
"This growing alliance of aggression shows why we must continue to stand steadfast behind Ukraine's fight for freedom," Mr Healey said.
The MoD said Russia and North Korea had been looking to deepen their partnership, with Russia's parliament agreeing a treaty including a mutual defence clause between the countries on 24 October.
MoD warns of 'interoperability difficulties'
However, Russian and North Korean forces would face "interoperability difficulties" as a result of not having trained together, Defence Intelligence added.
It said: "[North Korean] troops conducting combat operations would almost certainly have issues operating Russian equipment, integrating into Russia's command and control structure, and working around the language barrier with Russian forces."
North Korea will support Russia 'until the day of victory'
It comes after North Korea's foreign minister said it will support Russia in its war against Ukraine "until the day of victory".
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Thousands of North Korean troops are stationed near Ukraine's border and are set to enter combat in the coming days, US secretary of state Antony Blinken has said.
He said 10,000 soldiers have been deployed to Russia, with up to 8,000 in the Kursk region, indicating they would be used on the frontline.
The troops have been trained by Russian forces in artillery, drones and "basic infantry operations, including trench clearing", he added.