The United States dismissed as "ludicrous" Wednesday claims by Russian Foreign Minister Sergei Lavrov that it was funding or running an offensive in Ukraine as Washington and Moscow again traded barbs.

"I think many of the claims he made in his interview are ludicrous and they're not based in facts of what is happening on the ground," State Department spokeswoman Jen Psaki said.

Ukraine's going ahead with its May 25 presidential election without consulting first with pro-Russian voters "is very destructive for the country," Russian Foreign Minister Sergei Lavrov said Wednesday in an interview with state-controlled RT television.

He said Moscow would only "recognize something that would be based on the all-inclusive process", adding, "to call an election without finding some common ground with the east and the south of Ukraine I think is very destructive for the country."

Germany's foreign minister warned Wednesday that the crisis in Ukraine was getting "harder and harder" to resolve, as the most serious confrontation between Moscow and the West since the Cold War showed no sign of abating.

"Each day that passes... makes a solution harder and harder to reach," Frank-Walter Steinmeier said while on a trip to Moldova, urging Moscow and Kiev to seize the "opportunity" of a deal reached in Geneva last week to defuse the crisis.

Russia issued a sharp warning on Wednesday that it will strike back if its "legitimate interests" in Ukraine are attacked, raising the stakes in the Cold War-like duel with the United States over the former Soviet republic's future.

Meanwhile, Ukraine's interior ministry said the military has "liberated" the eastern town of Svyatogorsk from pro-Kremlin separatists.

The U.S. State Department said it was "deeply concerned" Wednesday over reports about the kidnapping of an American journalist by pro-Russian separatists in Ukraine.

"We are deeply concerned about the reports of a kidnapping of a U.S. citizen journalist in Slavyansk, Ukraine, reportedly at the hands of pro-Russian separatists," State Department spokeswoman Jen Psaki said in a statement.

Western leaders should set aside their differences with Russia over Ukraine to focus on the growing threat of Islamic extremism, former British prime minister Tony Blair said on Wednesday.

In a speech in London, the Middle East envoy said the spread of extremist ideology in that region as well as in Pakistan, Afghanistan and North Africa "represents the biggest threat to global security of the 21st century".

In the eastern Ukrainian city of Donetsk, the split between nationalists and pro-Russians extends to religion, with sharp divides appearing between churches tied to Moscow and those that want to keep the country united.

"We should say thank you to (Russian President Vladimir) Putin, who helped us to understand that we are Ukrainians. The aggression against Ukraine has helped reinforce our identity," said Father Tikhon, a 43-year-old priest of the local Greek Catholic church.

Russia issued a blunt warning Wednesday it would respond if its interests are attacked in Ukraine, as pro-Kremlin rebels in the restive east of the country braced for a new military offensive by Kiev.

The threat by Russian Foreign Minister Sergei Lavrov, recalling the 2008 war with Georgia over breakaway South Ossetia, came as U.S. troops were headed to region in a show of force after Washington again warned Moscow of new sanctions over the escalating crisis.

Ukraine's acting president on Tuesday ordered the military to resume an operation against pro-Kremlin separatists, after the "brutally tortured" bodies of two people, one a kidnapped local councilor, were discovered in the restive east.

"I demand the resumption of effective counter-terrorism measures to protect Ukrainian citizens in the east from terrorism," Oleksandr Turchynov said, according to a statement from his other office as parliamentary speaker.

The European Union called Tuesday on all sides to implement speedily the terms of the Geneva accord aimed at defusing the Ukraine crisis.

"We call on all sides to respect the terms and conditions of the agreement in Geneva," a spokesman for EU foreign affairs chief Catherine Ashton said.