"The farce, which terrorists call the referendum, will have no legal consequences except the criminal responsibility for its organizers," Ukraine's acting President Oleksandr Turchynov said in a statement.
While controversial, the vote gave momentum to the separatists and bolstered Russia's argument that easterners want more autonomy and deserve more say in running Ukraine.
Russian President Vladimir Putin's office voiced hope that the OSCE could help broker talks between Kiev and the two provinces. The cautious stance — which contrasted with Russia's quick annexation of Crimea after a separatist vote there — appeared to show Moscow favoring a negotiated solution.
"The practical implementation of the referendum results should proceed in a civilized way without any throwbacks to violence through a dialogue between representatives of Kiev, Donetsk and Luhansk," the Kremlin said.
Switzerland's Burkhalter, whose country currently chairs the OSCE, met with Putin last week and spoke with him again by phone Monday about a road map for settling the crisis.
"We have seen in Moscow that there is openness for a dialogue," Burkhalter said. He said Ukraine has accepted a proposal to nominate Ambassador Wolfgang Ischinger of Germany as the OSCE co-moderator for the talks.
Burkhalter outlined some of that plan Monday in Brussels, where the European Union's foreign ministers added 13 people and two firms to their visa ban and asset freeze list over Ukraine, according to two officials who spoke on condition of anonymity because the measure had yet to be officially announced.
The U.S. and the EU, which have slapped travel bans and asset freezes on members of Putin's entourage after Russia's annexation of Crimea, have warned they could target entire sectors of the Russian economy if Moscow tries to derail the May 25 vote.
U.N. Secretary-General Ban Ki-moon urged both sides "to find a way back to the spirit of compromise" exhibited on April 17 in Geneva when Russian Foreign Minister Sergey Lavrov, U.S. Secretary of State John Kerry and diplomats from Ukraine and Europe agreed on tentative steps to halt violence and calm tensions — and to implement the accord.
"There is still time halt the descent of Ukraine into full-blown conflict," Ban told reporters at U.N. headquarters.
But Lavrov said Moscow saw no need for another four-way meeting, saying Ukrainian authorities should focus on a dialogue with the east.
He accused Washington and Kiev of stonewalling the OSCE plan and warned that efforts to defuse the crisis wouldn't succeed without "engaging opponents of the regime in a direct dialogue."
The Foreign Ministry criticized the EU for its "inarticulate reaction" to the OSCE plan, and said by refusing to recognize the referendums, the EU "undermines its credibility as a partner" in resolving the crisis.
Last week, Putin urged referendum organizers to postpone the balloting in an apparent attempt to distance himself from the insurgents.
Sunday's balloting appeared to be mostly peaceful but armed men stopped the voting and took control of the town hall in Krasnoarmeisk, then opened fire on the crowd outside. They identified themselves as Ukrainian national guards but the Interior Ministry said they were not. Two deaths were reported.
Turchynov and Ukraine's caretaker government came to power in February following the ouster of Kremlin-friendly President Viktor Yanukovych after months of protests in Kiev. Moscow and many in Ukraine's east have accused the new government of intending to trample the rights of Russian-speakers.
Isachenkov reported from Moscow. Nataliya Vasilyeva and Mark Rachkevych in Kiev, Ukraine, Raf Casert in Brussels, Angela Charlton in Paris, Edith M. Lederer at the United Nations, and photographer Manu Brabo in Krasnoarmeisk contributed reporting.