Up to 3,000 Ukrainian troops have been killed since the start of the Russian war late February, the Ukrainian president said.
"As to our military, out of the numbers we have, we think that we lost 2,500 to 3,000, in comparison with the Russian military, who lost about 19 (thousand) to 20,000," Volodymyr Zelenskyy said in an interview with CNN on Friday.
Almost 10,000 others have been injured during the war and "it's hard to say how many will survive," Zelenskyy added.
Russia, however, said it has lost 1,351 soldiers during the war.
The Ukrainian leaders warned all the countries should be prepared for the possibility of Russia's use of tactical nuclear weapons in the war.
"Chemical weapons, they should do it, they could do it, for them the life of the people, nothing. … We should think not be afraid, not be afraid but be ready," Zelenskyy said.
Meanwhile, the president said it is difficult to talk about civilian casualties as towns and cities south of Ukraine are blocked.
"… Kherson, Berdyansk, Mariupol further east, and the area to the east where Volnovakha is -- we just don't know how many people have died in that area that is blocked."
Speaking about the sinking of Russia's Moskva missile cruiser on Friday, Zelenskyy said: "For us, it is a strong weapon against our country, so its sinking is not a tragedy for us."
Russia and Ukraine have offered varying accounts of how the Russian missile cruiser Moskva sunk in the Black Sea. Moscow claimed a fire broke out on board, leading to the explosion of munitions that were being carried. Ukraine claimed it struck the strategically valuable vessel in a missile attack.