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 eastern Donbas region.
President Vladimir Putin is due to proclaim the annexation of nearly a fifth 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."
Putin said last week he was willing to use nuclear weapons to defend Russia's "territorial integrity".
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 organised votes breached international law and were coercive and wholly unrepresentative.
The exact details of Russia's annexation are unclear but it appears that Russia is laying claim to about 109,000 sq km (42,000 sq miles) of Ukrainian territory, or about 18%, in addition to Crimea, which Russia annexed in 2014.
If Russia could establish control over the whole area it claims, Putin would have annexed around 136,000 sq km or more than 22% of Ukraine, whose borders Russia recognised in a treaty after the fall of the Soviet Union.
But Russian forces do not control all of that territory. Even as Putin prepared to officially announce the start of the annexation, Ukrainian forces encircled the Russian stronghold of Lyman in the north of Donetsk region.
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.
But Peskov was less clear about whether or not Russia would lay claim to the whole of Kherson and Zaporizhzhia regions, which were recognised by Putin on Thursday as independent states.
"We will clarify everything today," Peskov said. Russia currently controls about 70% of Zaporizhzhia region.
Luhansk and Donetsk, with a combined population of about 6 million before the invasion, are collectively known as the Donbas, a mostly Russian-speaking centre 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.
(Reporting by Reuters; editing by Guy Faulconbridge and Kevin Liffey)