Volodymyr Zelensky has warned Ukrainians to prepare for a possible Russian nuclear attack and called for a stockpile of radiation pills.
He told the world "we must prepare for" the bleak scenario and get hold of medicine which would help curb radiation sickness.
Zelkensky made the grim warning in an interview before sharing the clip on his Telegram channel on Saturday evening.
On Friday he made a similar announcement, and said nuclear weapons could could not be ruled out with Vladimir Putin considering the move as Russia's war with Ukraine stalls.
Putin ramped up his offensive after the sinking of the Moskva battleship on Thursday and attacked eight towns including Kyiv, Kharkiv and Lviv.
A British fighter, meanwhile, was paraded on TV after being captured by Russian forces in Mariupol.
Dmitry Peskov, a Kremlin spokesman, told CNN an interview Moscow would use a nuclear deterrent on Ukraine in the event of an "existential threat".
Russia's policy appears to be "escalate to de-escalate" and this could include launching a small nuclear weapon to gain the upper hand in the war, according to the nation's military doctrine.
Zelensky said: "We shouldn't wait for the moment when Russia decides to use nuclear weapons... We must prepare for that."
More air raid shelters and anti-radiation medicines would would be needed, he said. The Ukrainian president added the Russians "can use any weapon, I'm convinced of it".
Six Ukrainian news websites broadcast the interview.
On Thursday the Mirror reported how Russia's flagship in the Black Sea had been severely damaged after being hit by missiles, according to Ukrainian officials.
Unconfirmed reports say up to 500 Russian sailors who were aboard may have died.
The Kremlin claims the Moskva missile cruiser was seriously damaged after ammunition on board the ship blew up, but Ukraine says it was struck by two Neptune missiles.
There are conflicting reports from Ukrainian sources if the ship has been sunk.
Michael Petersen, director of the Russia Maritime Studies Institute at the US Naval War College, told the Radio 4's Today programme about the news that the Russian vessel the Moskva has been seriously damaged.
He then said it was too early to know what happened but it appears to look like the "Ukrainians have managed to pull off a not-insignificant victory at sea".
Mr Petersen said: "I think it's one that has symbolic and military value as well.
"This is the flagship of Russia's Black Sea fleet, and it's getting a bit long in the tooth but it is the symbol of Russian naval power in the Black Sea."
He went on to explain that the Ukrainians had lost most of their navy to the Russians during the annexation of Crimea in 2014, and that this attack was "important symbolically".
