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Cliff Owen, Associated Press
Ukrainian Foreign Minister Pavlo Klimkin speaks to reporters at the State Department in Washington, Tuesday, July 29, 2014, during a news conference with Secretary of State John Kerry. The United States says there's been "no shred of evidence" that Russia is willing to help end the violence and bloodshed between the Ukraine separatists backed by Moscow and the government in Kiev.

WASHINGTON — The United States on Tuesday said Russia has "not shown a shred of evidence" that it is willing to help end the violence between the Ukraine separatists backed by Moscow and the government that is trying to maintain its sovereignty.

Secretary of State John Kerry issued a new warning that the U.S. and European Union are prepared to slap additional sanctions on Russia if it continues to ship weapons, funds and fighters across its border to support the separatists in eastern Ukraine.

He said the fighting between the separatists and Ukraine's military has prevented investigators from obtaining necessary evidence — and even remaining body parts — from the crash site of Malaysia Airlines Flight 17, which was shot down nearly two weeks ago.

"While the Russians have said that they want to de-escalate the conflict, their actions have not shown a shred of evidence that they really have a legitimate desire to end the violence and end the bloodshed," Kerry said following a State Department meeting with Ukraine Foreign Minister Pavlo Klimkin.

Kerry cited "clear evidence" of Russian military support to the separatists, including rocket and artillery fire from Russia into Ukraine.

He also said he spoke with Russian Foreign Minister Sergey Lavrov on Tuesday morning about what he described as specific proposals to ease the violence.

But Kerry said the separatists will continue to fight "unless they feel some pressure, something real, from their Russian backers" to stop.

For months, the separatists in eastern Ukraine have waged a bloody battle against Kiev to secede from the country and align themselves with Moscow.

In Moscow, Russia's Foreign Ministry said that Lavrov had urged Kerry to persuade the Ukrainian government to declare a cease-fire.

But Kerry said it was the separatists who have refused to lay down their arms and try to find a political solution to the war.

"They have displayed an appalling disregard for human decency," Kerry said, adding that Russian President Vladimir Putin "can make a huge difference here — if he chooses to."

Klimkin said Kiev remains willing to negotiate a peace plan with the separatists that would include a bilateral cease-fire and a political settlement "with the aim of restoring the territorial integrity of Ukraine."

He thanked the U.S. for its efforts to broker peace. "I feel a real commitment here, a real solidarity here," Klimkin said.

Additionally, Kerry and Lavrov discussed the fulfillment of a Cold War-era arms control deal that the U.S. has accused Russia of violating.

Earlier this week, Washington said Moscow violated the 1987 Intermediate Range Nuclear Forces (INF) Treaty by testing a new ground-launched cruise missile.

The Russian Foreign Ministry didn't comment on the accusations in releasing details about the Tuesday discussion between Kerry and Lavrov.

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