Russia has threatened nuclear retaliation if Ukraine mounts a successful counter-offensive.
Ex-president Dmitry Medvedev, who is deputy chairman of Russia’s Security Council, warned of “global nuclear fire” if Kyiv forces and Nato seized any Russian land.
He hit out after an alleged Ukrainian drone attack on Moscow in the early hours of Sunday, with two crashing into offices just close to the Kremlin, shaking the financial district of the city.
The strikes, which temporarily shut down Vnukovo international airport, happened at 3.20am and 4.10am.
No fatalities have been reported but photos from the scene show damaged windows at the corner of the buildings, with debris scattered on the ground below.
Moscow blamed the attack on Ukraine but Kyiv officials have not taken responsibility.
Medvedev said: “Imagine if the.. offensive, which is backed by NATO, was a success and they tore off a part of our land then we would be forced to use a nuclear weapon according to the rules of a decree from the president of Russia.
“There would simply be no other option. So our enemies should pray for our warriors’ (success). They are making sure that a global nuclear fire is not ignited.”
Ukraine’s President Volodymyr Zelensky on Sunday night warned the “war is returning to the territory of Russia” after the drone attack.
He said: “Today is the 522nd day of the so-called ‘Special Military Operation’, which the Russian leadership thought would last a couple of weeks. Gradually, the war is returning to the territory of Russia - to its symbolic centres and military bases, and this is an inevitable, natural and absolutely fair process.”
It came after Vladimir Putin said he did not dismiss the idea of peace talks following a meeting with African leaders in St Petersburg.
The Russian leader claimed a joint African and Chinese initiative could serve as a basis for finding peace.
He told reporters: “The initiative, in my opinion, can be the basis of some processes aimed at the search for peace, the same as others, such as for example, the Chinese initiative.”
Putin said it was hard to implement a ceasefire when the Ukrainian army was on the offensive.
Mr Zelensky has rejected the idea of a ceasefire now, saying it would leave Russia in control of nearly a fifth of his country and give its forces time to regroup after 17 grinding months of war.
