Washington has denounced what it calls "contrived and bogus" plans by pro-Russia separatist groups in eastern Ukraine to conduct a weekend referendum on self-determination.
The pro-Russia separatists that are planning the referendum are pushing for autonomy and independence for parts of eastern Ukraine -- a situation similar to a regional referendum in Crimea in March that led to Russia's annexation of the peninsula.
Speaking after meeting in Washington on May 6 with European Union foreign policy chief Catherine Ashton, U.S. Secretary of State John Kerry said: "We flatly reject this illegal effort to further divide Ukraine. Its pursuit will create even more problems in the effort to try to de-escalate the situation."
Kerry described regional votes in the eastern Ukrainian cities of Donetsk and Luhansk as "the Crimea play-book all over again," saying "no civilized nation is going to recognize the results of such a bogus effort."
Although Russia has de facto control over Crimea, few nations recognize the move.
The United States and Europe have imposed sanctions against Russians linked to President Vladimir Putin's inner circle and against pro-Russia Ukrainians who were involved in Russia's annexation.
Sanctions also have been imposed over Moscow's continued destabilizing actions in the mainly Russian-speaking east Ukraine.
U.S. officials have said that all Ukrainians should vote on their future governance in nationwide elections set for May 25.
Ashton said after her talks with Kerry on May 6 that "It's for the people of Ukraine to decide what Ukraine is and will be, and they will consider very carefully where they sit in the world."
Meanwhile, both Kerry and Ashton criticized Russia for failing to abide by an agreement reached in Geneva in April aimed at de-escalating tensions in eastern Ukraine.
They threatened that any move by Moscow to annex additional parts of Ukraine would be met by more sanctions.
Such sanctions, they said, would go beyond penalties already imposed on Russian officials and executives – and would be applied to entire sectors of the Russian economy, including the defense industry, finance, and energy.
In Ukraine on May 6, Ukrainian troops tightened a security cordon around the separatist-held eastern city of Slovyansk.
Pro-Russia militia fighters acted with apparent impunity elsewhere in the turbulent region bordering Russia, surrounding a key Interior Ministry base in the area.
With reporting by Reuters, AP, and AFP