The Ukrainian separatist region of Donetsk appealed to Moscow on Monday to consider its absorption into the Russian Federation to "restore historic justice".
The statement, the day after a referendum on self-rule that rebels said had mustered some 80 percent support, was certain to infuriate Kiev and anger Western states who believe the rebellion has been engineered by the Kremlin.
The Ukrainian Prime Minister's office said it had no immediate reaction to the statement by Denis Pushilin, a leading member of the self-declared "Donetsk People's Republic", at a news conference in largely Russian-speaking Donetsk.
He said Belarus also belonged in a broader union of states that once formed the core of the Soviet Union.
"Based on the will of the people and on the restoration of an historic justice, we ask the Russian Federation to consider the absorption of the Donetsk People's Republic into the Russian Federation," he said.
"The people of Donetsk have always been part of the Russian world. For us, the history of Russia is our history."
Rebel leaders have seized control of a string of cities across the east of the country and fighting has worsened over the last week.
Kiev and Western states accuse Russia, which in March annexed the Crimean peninsula from Ukraine after a referendum, of backing the rebellion with special forces. Moscow denies this.
Pushilin said Ukraine, Belarus and Russia - states that formed the core of the now defunct Soviet Union - belonged in a restored historic union. Hopes of realizing this were dashed by the emergence of a "junta" in Kiev after the overthrow of pro-Russian president Viktor Yanukovich.
Russian President Vladimir Putin has suggested Bolshevik revolutionaries drew up arbitrary borders for what were the Soviet republics of Ukraine and Russia, and many Russians regarded Soviet leader Nikita Khrushchev's handover of Crimea to Ukraine in 1954 as an historic injustice.
"Terror followed, taking hundreds of lives in Slaviansk, Odessa, Mariupol and other cities in Ukraine," he said, naming cities that have been at the centre of clashes between Ukrainian forces and rebel fighters in recent weeks.