The North Korean troops being prepared to fight in Ukraine include land forces and “other technical personnel”, Zelenskyy claimed.
Ukraine’s allies have yet to back the Ukrainian assertion that North Korea is sending troops although they say they are studying it.
White House National Security Council spokesman Sean Savett said on Tuesday any North Korean troop involvement in Ukraine, if true, would mark a significant increase in the North Korea-Russia defence relationship.
Zelenskyy said Russia needs to fill the gap in mobilisation and cover its own personnel losses.
Ukraine and its allies say hundreds of thousands of Russian soldiers have been killed or wounded fighting in Ukraine while Ukraine’s own military losses – a closely guarded secret – are also high but smaller.
Russian forces have advanced over the past several months in Ukraine’s east.
Ukraine has called on allies to respond firmly to North Korean aid for Russia, including by imposing new sanctions and further isolating North Korea.