Russian-backed rebels fighting against Kiev announced on Tuesday a plan to create a new "state" which they said would take the place of Ukraine and have its capital in rebel territory.
Ukraine's pro-western authorities immediately ridiculed the idea as a Kremlin project which they would never allow to get off the ground.
The separatists said the proposed country would be founded after a referendum and called Malorossiya, a tsarist-era name meaning "Little Russia" which once described most of the area covering modern-day Ukraine.
A constitution presented by rebel leader Alexander Zakharchenko said his self-declared Donetsk People's Republic, neighbouring rebel-held Luhansk and other regions had agreed to "declare the establishment of a new state, which is the successor of Ukraine".
The document — released by the separatists' news agency — said rebel bastion Donetsk would become the capital, while Kiev would be reduced to the status of a "historical and cultural centre".
Ukraine's foreign minister, Pavlo Klimkin, quickly derided the plan on Twitter as another "show" by the insurgents' "Kremlin puppet masters".
"We and our partners will not let this happen," he vowed.
Meanwhile, the country's top military commander, Viktor Muzhenko, said on social media that the Ukrainian people would "bury" Malorossiya, calling the plan one of the rebels' "sick fantasies".
The proposal seems to stand no chance of gaining traction and even the insurgents appeared unable to agree on it.
The press service for Luhansk rebel chief Igor Plotnitsky said he had not been consulted on the project.
The surprise announcement of Malorossiya could, however, further dent an already stalled peace process which has failed to end more than three years of fighting. Some 10,000 people have been killed in that time.
A deal brokered by the leaders of Ukraine, Russia, France and Germany in Misk in 2015 has hit a wall but is still viewed by those involved as the only way of unwinding Ukraine's war.
France's foreign ministry called on Moscow "to denounce" the rebel announcement, which it described as a "violation" of the peace deal.
"Russia has to intensify its efforts to put an end to this conflict," the ministry said.
The Kremlin, which denies allegations it controls the rebels, was expected to comment on the matter later on Tuesday.
Three former rebel leaders said in May that a top aide to Russian president Vladimir Putin decides how the pro-Moscow administration of eastern Ukraine is run and who gets what jobs there, challenging Kremlin denials that it calls the shots in the region.