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Announcing the result of the vote in one of the two provinces where it was held, a leader of the "People's Republic of Donetsk", Denis Pushilin, said it was now an independent state and would appeal to join the Russian Federation.
"The people of Donetsk have always been part of the Russian world. For us, the history of Russia is our history," he said.
"Based on the will of the people and on the restoration of a historic justice, we ask the Russian Federation to consider the absorption of the Donetsk People's Republic into the Russian Federation," he told a news conference.
In neighbouring Luhansk, officials said they might now hold a second referendum on joining Russia, similar to one held in Crimea, a Ukrainian region Moscow seized and annexed in March.
Donetsk and Luhansk together are home to 6.5 million people and produce around a third of Ukraine's industrial output, creating the biggest new self-proclaimed independent states in Europe since the break-ups of Yugoslavia, Czechoslovakia and the Soviet Union itself more than 20 years ago.
Donetsk separatists said more than 80 percent of voters had supported independence. Those in Luhansk said more than 96 percent did.
The government in Kiev and its Western backers say the exercise was absurd, with no legal basis, insecure polling stations, old voter lists, ballots that could be easily reproduced and self-proclaimed election officials openly promoting secession. They say residents who support a united Ukraine were most likely to have stayed home out of fear of rebel gunmen and to avoid lending the vote credibility.
Unlike in Crimea, Moscow has stopped short of recognising the two regions as independent from Kiev and has said nothing to suggest it would endorse their absorption into Russia. President Vladimir Putin even called last week for the referendum to be postponed.
But Moscow indicated clearly on Monday that it intends to use the results of the referendums to put pressure on the government in Kiev to recognise the rebels in the east as a legitimate side in talks.
"We believe that the results of the referendum should be brought to life within the framework of dialogue between Kiev, Donetsk and Luhansk," the Russian Foreign Ministry said.
It accused the Kiev government of a "criminal lack of readiness for dialogue with their own people".
The Russian stance appears calculated to entrench Moscow's allies in control of Ukraine's industrial heartland without taking the sort of overt steps - sending in ground forces or formally recognising the regions' split from Kiev - that might invite tough sanctions from the West.
The mayor of Slaviansk, a small city in the Donetsk region that has become the most heavily fortified rebel redoubt, said Ukrainian troops were now occupiers, and Russian troops should be invited to help defend the area.
"They should go," Vyacheslav Ponomaryov said of Ukrainian forces. "We're going to defend our territory."
As for bringing in Russian forces: "I support this. We need Russian troops to provide stability and a peaceful life in the region's future."
The European Union added the names of some individuals and firms on Monday to a list of those facing asset freezes and travel bans, measures Moscow has long mocked.
But both Brussels and Washington have so far stopped short of wider sanctions designed to hurt Russia's economy more broadly, despite repeated threats.
The United States and European Union both said they would not recognise the results of the "illegal" referendum.
However, in the latest sign that the West is not ready to impose more serious economic measures, diplomatic sources said France would press ahead with a 1.2 billion-euro contract to sell helicopter carrier ships to Russia because cancelling it would hurt Paris more than Moscow.
Losing control of Donetsk and Luhansk would be a crippling blow for Ukraine, a country of around 45 million people the size of France, facing bankruptcy after half a year of turmoil.
Donetsk and Luhansk produce more than 15 percent of Ukraine's GDP, including around a third of its industrial output from the giant steel smelters and other heavy industry of the Donbass, one of Europe's most productive coal producing regions.
If they slip out of Kiev's control without being formally absorbed by Moscow, they would become by far the biggest and most economically important of the self-proclaimed independent statelets Russia protects in other parts of the ex-Soviet Union.
Since the early 1990s Russian troops have protected breakaway statelets in a sliver of Moldova and two parts of Georgia, but all three of those regions combined have barely an eighth of the population of Donetsk and Luhansk.
The International Monetary Fund, which is arranging a bailout of Ukraine's finances, has said it would have to renegotiate if Kiev lost control of the east.
The government in Kiev and Western nations accuse Russia of stirring up unrest in the east following the overthrow of a pro-Moscow president in February by protesters demanding closer links with Europe.
In March, Putin overturned decades of post-Cold War diplomacy by announcing Russia's right to intervene in Ukraine, seizing and annexing Crimea and massing tens of thousands of troops on the frontier. Putin said last week he had withdrawn the troops from the border area, but Washington and NATO said this was not true. They also say Russian special forces are active on the ground, which Moscow denies.
Ukraine's acting President Oleksander Turchinov accused Russia of working to overthrow legitimate state power in Ukraine. He said the Kremlin was trying to disrupt a Ukrainian presidential election later this month.
Eastern Ukraine has been plagued by turmoil as Kiev has staged a largely failed military operation to regain control of towns held by the separatists. Authorities said 49 people have been killed in violence in the region of Donetsk since March 13.
The rebels have given differing accounts of their precise plans. However, participating in the Ukrainian presidential election on May 25 is clearly ruled out.
"As of today, we are now the Republic of Luhansk, which believes it to be inappropriate and perhaps even stupid to hold a presidential election," RIA cited a spokesman for rebels in that region as saying.
Some rebels have publicly supported pressing for annexation by Russia.
"This land was never Ukraine ... We speak Russian," said Ponomaryov, mayor in Slaviansk.
Asked about the possibility of holding a second referendum, on union with Russia, he said: "There has been no decision, but this referendum showed we are prepared ... We can put on an election or referendum at short notice at barely any cost."
Ukrainian Interior Minister Arsen Avakov said rebels had made a new attempt overnight to seize a television tower on the edge of Slaviansk. Eastern regions of Ukraine receive Russian state television, which has broadcast relentless accounts of a threat from "fascists" in Kiev. Ukraine has struggled to keep its own television stations on the air in the region.
"The information war that they are waging against us in the Donbass is more dangerous than a bullet," he wrote on Facebook.
But there was hint of compromise in the port of Mariupol, scene of fierce fighting between Ukrainian forces and rebels over the last week. Turchinov said police had begun patrols with a volunteer militia set up by Metinvest, a firm mostly owned by Ukraine's wealthiest businessman, Rinat Akhmetov.