Several Russian planes that used to carry the country’s nuclear weapons have been spotted flying within striking distance of Ukraine.
The aircraft are believed to be Russian TU-95s, known as Bears, and were seen over the Kaluga region between Moscow and the Ukraine border.
Videos caught four of the aircraft flying within striking distance of possible targets in Ukraine.
Moscow’s defence ministry has not confirmed the purpose of the flights.
The propeller-powered Bears have been used previously to strike targets in Ukraine with non-nuclear weapons such as Kh-55 and Kh-101 air-launched cruise missiles.
The super-loud Tu-95 is the only propeller-powered strategic bomber still in operational use today, and the plane first flew 70 years ago.
Vladimir Putin has ramped up his offensive after the humiliating sinking of the Moskva battleship on Thursday.
The ship went down in the Black Sea after Kyiv said it launched a rocket strike , although the Kremlin claims the vessel was destroyed in an accident.
Around 500 sailors were said to have been aboard the flagship missile cruiser.
In retaliation eight towns were attacked, including Kharkiv and Lviv.
The emergence of the videos come less than a day after Volodymyr Zelensky 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.
Zelensky 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 not be ruled out with Vladimir Putin considering the move as Russia's war with Ukraine stalls.
The Ukrainian president said: "We shouldn't wait for the moment when Russia decides to use nuclear weapons... We must prepare for that."
He added he is “convinced” Putin could use any weapon against his people.
Last month Moscow said it would only use nuclear weapons if the nation's existence is under threat.
Russia ’s nuclear forces were put on high alert in the early days of the war.
Putin ordered the Russian defence minister and the chief of the military's General Staff to put the nuclear deterrent forces in a "special regime of combat duty".
The US Ambassador to the United Nations said at the time Putin's order shows the Russian leader is "escalating conflict in a manner that is unacceptable".
Linda Thomas-Greenfield said: "It means that President Putin is continuing to escalate this war in a manner that is totally unacceptable.
"We have to continue to stem his actions in the strongest possible way."
