U.S. Secretary of State John Kerry has urged Russia not to interfere in the upcoming presidential poll in Ukraine on May 25, warning that Moscow risks more targeted economic sanctions.

Speaking in London after a meeting with foreign ministers from allied European countries on May 15, Kerry said the message for Russia: "Let Ukraine vote."

"If Russia or its proxies disrupt the [Ukrainian presidential] elections, stand in the way of the Ukrainian people being able to exercise their vote, that is when and if there would be additional sanctions," he said.

"I'm not going to get into announcing today what the sanctions are," Kerry said. "We have completed our work. We know what they [sanctions] are... If they have to go into effect, they will have an impact."

However, he said he still hopes Russia will back the voting in Ukraine and will make separatists to stick to the conditions, which have been earlier supported by Russia.

"Let me emphasize, our hope is not to do this [apply rougher sanctions]. I say to the Russians and everybody, our hope is to de-escalate," Kerry stressed.