People react during a rally to mark and celebrate announcement of results of the referendum on the status of Donetsk region in Donetsk May 12, 2014. [Photo/Agencies]

White House spokesman Jay Carney said Monday the United States does not recognize the results of the vote, and is focusing on making sure Ukraine's presidential election take place as planned.  

Switzerland's Burkhalter, whose country currently chairs the OSCE, met with Putin last week and spoke with the Russian leader again by phone Monday about resolving the Ukrainian 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 his 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 US 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 Ukraine's presidential vote.

Russian Foreign Minister Sergey Lavrov said Moscow saw no need for another four-way meeting between Russia, the United States, the EU and Ukraine following their talks in Geneva last month, saying that the Ukrainian authorities should now focus holding a dialogue with the east.

The Foreign Ministry criticized the EU for its "inarticulate reaction" to the OSCE plan, and said by refusing to recognize the referendum Sunday, the EU "undermines its credibility as a partner" in resolving Ukraine's crisis.