Dmitry Medvedev, a former Russian president, also says there is no going back on referendums being organised by Russia-appointed and separatist authorities in large swathes of Ukraine.
Former Russian president Dmitry Medvedev has said that any weapons in Moscow's arsenal, including strategic nuclear weapons, could be used to defend territories incorporated in Russia from Ukraine.
Medvedev, deputy chairman of Russia's Security Council, said on Thursday that referendums being organised by Russia-appointed and separatist authorities in large swathes of Russian-controlled Ukrainian territory will take place, and that "there is no going back".
"The Donbass (Donetsk and Luhansk) republics and other territories will be accepted into Russia."
Medvedev said the protection of all the territories would be significantly strengthened by the Russian armed forces.
"Russia has announced that not only mobilisation capabilities, but also any Russian weapons, including strategic nuclear weapons and weapons based on new principles, could be used for such protection," he added.
The referendums will take place in the Russian-held parts of Ukraine's Donetsk, Luhansk, Kherson and Zaporizhzhia provinces, as well as part of Mykolaiv province.
The votes, being organised at a few days' notice under military occupation, have been labelled shams by Kiev and Western countries.
If formally admitted to the Russian Federation, those territories will under Moscow's nuclear doctrine be entitled to protection from Russian nuclear weapons.
Moscow does not fully control any of the four regions it is expected to try to annex, with only around 60% of Donetsk and 66% of Zaporizhzhia regions held by the Russian army.