Ukrainian President Volodymyr Zelenskyy said Thursday his government has intelligence information that 10,000 troops from North Korea are being prepared to join Russian forces fighting against his country, warning that a third nation wading into the hostilities would turn the conflict into a “world war.”

Zelenskyy did not go into detail about the claim that came a day after U.S. Deputy Secretary of State Kurt Campbell said that Washington and its allies are alarmed by North Korea’s military support for Russia’s war in Ukraine but couldn’t confirm Ukrainian claims that soldiers were sent to fight for Moscow.