President Volodymyr Zelenskyy claimed that Russia is in the process of deploying 10,000 North Korean soldiers to fight against Ukraine.
Zelenskyy has previously accused North Korea of sending troops to Russia's army but this was the first time he gave an exact figure.
He said that based on "information from our intelligence" an unspecified number of North Korean soldiers were already on "occupied Ukrainian territory from the side of Russian enemies."
"We know (of) about 10,000 soldiers of North Korea that they are preparing to send fight against us," he added, speaking to reporters in Brussels after talks with EU leaders.
Western officials said they were aware of the reports but were treating them with caution at this stage.
"We are tracking the possible deployment of North Korean troops to Russia," one official told reporters.
"North Korea has for quite some time now provided significant artillery munitions to Russia to support the war. And this is a new body of reporting that we're tracking."
The official said the reports concerned between 2,000 and 12,000 North Koreans but that -- if verified -- "it's probably towards the lower number".
"In terms of the veracity, it is a little bit too early to tell, to get some ground truth behind actually what is happening here," the official said.
The Ukrainian leader attended an EU leaders' summit and a meeting of NATO defense ministers in Brussels as he pressed for support for his "victory plan" to end the war against Russia.
Zelenskyy said the North Korean troops included "land forces" and "other tactical personnel."
"This is the first step to a world war," he warned, noting that Iran was also backing Russia with "drones and missiles", a claim that Tehran has repeatedly denied.
He said Russian President Vladimir Putin was "counting" on the North Korean soldiers because he was "afraid of mobilization."
Zelenskyy conducted a whirlwind tour of Western capitals earlier this month including Washington, Paris, Berlin, Rome and London to promote his initiative.
Experts have long said North Korean missiles are being deployed in Ukraine by Russian forces. Moscow and Pyongyang have denied this.
Putin made a rare visit to Pyongyang in June, when he signed a mutual defense agreement with leader Kim Jong Un.
Officials also claim Pyongyang has sent thousands of containers of weapons to Russia for use in Ukraine.
Moscow and Pyongyang have been allies since North Korea's founding after World War II and have become closer since Russia's 2022 invasion of Ukraine.
