The Kremlin said on Friday that attacks against any part of the swathe of Ukraine that President Vladimir Putin was about to annex would be considered aggression against Russia itself, adding that Russia would fight to take the whole of the Donbas region.
President Vladimir Putin is due to proclaim the annexation of more than 15% of Ukraine on Friday, escalating his seven-month war and taking it into an unpredictable new phase.
Moscow is declaring Ukraine's Donetsk, Luhansk, Kherson and Zaporizhzhia regions, largely or partly occupied by Russian or Russian-backed forces, to be part of Russia.
Asked by reporters if an attack by Ukraine on the territories Russia is claiming as its land would be considered an attack on Russia, Kremlin spokesman Dmitry Peskov said: "It would not be anything else."
Russia is moving to annex the regions after holding what it called referendums in the occupied areas of Ukraine. Western governments and Kyiv said the hastily organized votes breached international law and were coercive and wholly unrepresentative.
While Russia controls almost all of Luhansk, claimed by Russian-backed separatists as the Luhansk People's Republic, it has control of only around 60% of the Donetsk region. The self-styled separatist Donetsk People's Republic, backed by Russia, claims the whole of Ukraine's Donetsk province.
Asked what would happen to the territory not under Russian control, Peskov said: "It is to be liberated."
He said the whole of the Donetsk region would become part of Russia.
Luhansk and Donetsk, with a combined population of about 6 million before the invasion, are collectively known as the Donbas, a mostly Russian-speaking center of coal mining and heavy industry until their economies were wrecked by the fighting from 2014 onwards.
The conflict in eastern Ukraine began in 2014 after a pro-Russian president was toppled in Ukraine's Maidan Revolution and Russia annexed Crimea.