WASHINGTON, May 6 -- The United States rejects the efforts of pro-Russia separatists in eastern Ukraine to organize a "contrived and bogus" independence referendum on May 11, Secretary of State John Kerry said on Tuesday.

"We flatly reject this illegal effort to further divide Ukraine. Its pursuit will create even more problems in the efforts to try to deescalate the situation," Kerry told reporters after meeting with EU foreign policy chief Catherine Ashton.

"This is really the Crimea playbook all over again and no civilized nation is going to recognize the result of such a bogus effort," Kerry added.

Russia annexed Crimea on March 18 following an independence referendum in the southern Ukraine peninsula that was not recognized by the West.

U.S. President Barack Obama warned of broader sanctions against Russia if it disrupts Ukraine's presidential elections scheduled on May 25, threatening to target its energy, arms and finance sectors.

If Russia takes the next step to "reenact its illegal Crimea annexation" in Ukraine and sends more forces over the border, harsh U.S. and EU sanctions will follow, State Department spokeswoman Jen Psaki said at a press briefing on Tuesday.

In a congressional hearing on Tuesday, Assistant Secretary of State Victoria Nuland told lawmakers that it is "in the U.S. national security interest" that Ukraine's presidential election reflects the will of its 45 million people.

"Free, fair elections on May 25th are the best route to political and economic stability in Ukraine," said Nuland, the top U.S. diplomat for European affairs.