The United States said Russia has "not shown a shred of evidence" that it is willing to help end the violence between the Ukraine separatists and pro-Russian rebels on Tuesday, Reuters reported.

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, according to Reuters.

Kerry said the fighting between the separatists and Ukraine's military has prevented investigators from obtaining necessary evidence, including remaining body parts, from the crash site of Malaysia Airlines Flight 17 shot down nearly two weeks ago, Reuters reported.

"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 Ukrainian Foreign Minister Pavlo Klimkin, according Reuters.

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

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

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, Reuters reported.

"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," according to Reuters.