Separatists in eastern Ukraine proclaimed the birth of two new “sovereign” republics Monday after asserting victory in controversial self-rule referendums, and one of the regions promptly asked to join Russia.

Leaders of the self-declared Donetsk People’s Republic also demanded that Ukrainian security forces leave the separatists’ territory.

The statements represented a hardening of positions that could drag Ukraine closer to all-out civil war and is likely to intensify tensions between Russia and the West.

“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,” Denis Pushilin, a separatist leader, said at a news conference.

[See Timeline: Key events in Ukraine’s crisis.]

Leaders of the parallel Luhansk People’s Republic told a crowd in the region’s capital that they proposed to join with Donetsk to form a new republic called “Novorossiya,” or New Russia, Ukrainian news media reported. According to a spokesman, the leaders were also considering whether to stage a second referendum to ask people outright if they wanted to join Russia.

Russia responded cautiously, repeating an earlier call for negotiations within Ukraine.

“We reaffirm the need for the immediate establishment of a broad discussion in Ukraine concerning its future state structure, involving all political forces and the country’s regions,” the Russian Foreign Ministry said in a statement.

Ukraine’s government has denounced the referendums as a “criminal farce” arranged by a “gang of Russian terrorists,” but it says it is willing to talk with regional leaders about autonomy.

The sentiments expressed in the referendums underline the ­urgent need for the Kiev government to negotiate with the separatists, perhaps offering meaningful autonomy, before the situation slips further out of its control, experts said.

“There are a very large number of people who are resentful towards Kiev,” said Adam Swain, a professor at the University of Nottingham in England and a frequent visitor to the region, adding that the Ukrainian government ignores these sentiments at its peril. “If Kiev wants to have any semblance of control in the region, they have no option but to start negotiating. It is crazy for them to reject this out of hand.”

The twin referendums — deemed illegal by the United States and the European Union — asked voters whether they supported “self-determination” for the Donetsk and Luhansk people’s republics, respectively. But separatist leaders were deliberately vague about what that meant, saying that the question of whether to seek federal autonomy within Ukraine, independence or absorption into Russia would be left to a later date.

Separatists said 89 percent of the people who voted in Donetsk and 96 percent of voters in Luhansk supported self-determination. The results could not be independently verified, and the way the referendums were administered — by the separatists themselves — lacked international credibility. Yet the vote appeared to reflect genuine and widespread mistrust of the interim government in Kiev, which came to power in February after pro-Russian President Viktor Yanukovych fled in the face of popular protests.

Most people who opposed the referendums simply stayed away from polling stations. Many of those who voted yes said they wanted to remain with Ukraine but had turned out to express anger at Kiev, especially for the deaths of pro-Russian activists and unarmed citizens during clashes in the port cities of Odessa and Mariupol.

Demand for allegiance

Once they had secured what they considered to be a popular mandate, separatists in Donetsk did not take long to reveal their true intentions.

In the rebel stronghold of Slovyansk, a Russian who portrays himself as the rebels’ military commander demanded the departure of Ukrainian security forces.

“All the soldiers and officers of the armed forces, internal security forces, the Security Service, the Interior Ministry and other paramilitary structures of Ukraine from now on are considered to be illegally within the territory of the Donetsk People’s Republic (DPR),” said a statement attributed to rebel military commander Igor Strelkov and distributed in pamphlets in Slovyansk and Kramatorsk. “Within 48 hours they are required to swear allegiance to the DPR or leave the country.”

E.U. documents and Ukrainian authorities identify Strelkov as a member of the Russian military intelligence agency GRU.

The authenticity of the pamphlets could not be independently verified, but separatist leaders in Donetsk also said that one of their main priorities was to fight representatives of the Kiev government. “We will propose they either shift to the people’s side, or ask them to leave our territory,” Pushilin said.

Crimea, an autonomous Ukrainian region with a majority ethnic Russian population, voted in a hastily called referendum in March to join the Russian Federation. That vote was followed by a declaration of independence, then Russia’s swift annexation of the region.

But if Russia tried to take over eastern Ukraine as well, full-scale war could erupt, because the region is not as solidly pro-Russian as Crimea, and Ukrainian military forces have shown signs of being more determined to fight back.

Separatist leaders in Donetsk and Luhansk were adamant that Ukrainian presidential and mayoral elections scheduled for May 25 would not take place on what they now consider their territory.

Russian news agency RIA Novosti said Luhansk may hold a referendum on joining Russia. “If this decision is taken, then, respectively, the will of the people will be taken into account,” the agency quoted a spokesman for the separatists as saying.

In Donetsk, Pushilin said he did not trust the Kiev government, blaming it for the breakdown of a Geneva accord on defusing tensions and for the deaths of pro-Russian activists in Odessa this month, which he said “had reduced the window of opportunity for negotiations.”

“The only topic to negotiate with Kiev is over swapping hostages,” he said, adding that any other subjects would require international mediation.

With fears mounting of a spiral into civil war, the Kremlin said the implementation of the vote results should be carried out “in a civilized way,” without violence.

The Kremlin statement appeared to be aimed at lending legitimacy to the referendums, saying that Russia respects “the will of the population of the Donetsk and Luhansk regions” and noting “high voter turnout despite attempts to disrupt the voting.”

But the vote seemed to further widen divisions between Russia and the West.

Pressure on gas supplies

Russia’s state-backed gas company, Gazprom, warned Monday that it would cut off supplies to Ukraine as early as June 3 if the Kiev government did not start paying for the gas in advance.

The threat was the most explicit sign yet that Russia is willing to use gas supplies as a tool to exert pressure on Ukraine. Such a cutoff would also throttle energy supplies to the rest of Europe, because a significant portion of the natural gas that the continent uses passes through Ukraine. However, demand for the fuel is significantly lower in the summer than in the winter, softening the impact of a June cutoff.

“It’s time to stop messing around,” Russian Prime Minister Dmitry Medvedev said in a meeting with Gazprom chief executive Alexei Miller and Russian Energy Minister Alexander Novak that was broadcast on Kremlin-backed news channels. “I think Gazprom has taken all possible steps to resolve this matter by other means.”

Russia cut off natural gas shipments to Ukraine in 2006 and 2009, severely hampering supplies to the rest of Europe, which gets about 15 percent of its natural gas from pipelines that pass through Ukrainian territory. Gazprom says Ukraine owes it $3.5 billion for gas shipments in 2013 and this year.

But Ukrainian authorities have contested that figure, saying that the gas company is charging extortionate, politically motivated rates. Gazprom last month nearly doubled its prices for gas sent to Ukraine, setting them higher than in any E.U. nation.

Earlier Monday, E.U. foreign ministers expanded sanctions over Russia’s actions in Ukraine, adding two Crimean companies and 13 people to the bloc’s list, the Reuters news agency reported ahead of a meeting in Brussels.

The sanctions are in addition to measures affecting 48 Russians and Ukrainians, who have been targeted with E.U. asset freezes and visa bans since Russia annexed Crimea in March.

British Foreign Secretary William Hague said before the meeting that it was essential to show Moscow that the E.U. is ready to step up measures “depending on Russia’s attitude” toward Ukraine’s May 25 elections.

Meanwhile, the German government has been spearheading a move to have the Organization for Security and Cooperation in Europe (OSCE) launch talks between rival factions in Ukraine. German government spokesman Steffen Seibert said the first such “roundtable” discussions were scheduled to begin Wednesday, with participants including representatives from the Kiev government, the Ukrainian parliament and Ukraine’s regions.

But Seibert did not specify which groups in the regions had agreed to meet, or, importantly, whether they would include any of those who organized the referendums in Donetsk and Luhansk.

German Foreign Ministry spokesman Martin Schäfer said the OSCE has appointed a former German diplomat, Wolfgang Ischinger, to moderate the negotiations. He said Foreign Minister Frank-Walter Steinmeier was heading to Kiev and eastern Ukraine for talks Tuesday and would travel to France later that evening to discuss the crisis with his counterpart in Paris, Laurent Fabius.

Polls have indicated that most residents of eastern Ukraine would prefer to remain part of that country.

Still, many in eastern Ukraine are deeply unhappy with the Western-leaning government in Kiev. They consider it illegal and in league with ultranationalist groups, and some worry that the large population of Russian speakers living in the east will be treated as second-class citizens. Their fears have been magnified by aggressive Russian propaganda.

Birnbaum reported from Moscow,
and Kunkle reported from Kiev. Anthony Faiola in Berlin, Daniela Deane in London and Alex Ryabchyn in Donetsk contributed to this report.