US Secretary of State John Kerry has accused Russia of distraction, deception and destabilisation in eastern Ukraine.
In a strongly worded statement, he called on Moscow to help defuse the crisis there or face further sanctions.
Earlier, Russia called on the US to make Kiev stop raids on pro-Moscow separatists in the east.
Russia ordered new military exercises on its border following the raids, drawing condemnation from Ukraine.
Moscow has tens of thousands of troops along its side of the border and on Friday the acting Ukrainian Defence Minister Mikhail Koval told the Interfax-Ukraine agency that they had come within a kilometre of the border.
The news comes amid an increasing war of words between the US and Russia over events in eastern Ukraine.
Pro-Russian separatists are occupying key buildings in a dozen eastern Ukrainian towns, defying the central government.
Mr Kerry praised the interim authorities in Kiev, saying they had honoured the agreement struck in Geneva on 17 April to de-escalate the crisis.
But he said Russia had put its faith in distraction, deception and destabilisation and said they had failed to call for separatists to leave official buildings and give up their arms as stipulated by the Geneva agreement.
He also accused Russian media of promoting President Vladimir Putin’s fantasy about events in Ukraine.
Mr Kerry said US intelligence was confident that Russia was playing an active role in destabilising eastern Ukraine with personnel, weapons, money and operational planning.
He added that the US was ready to impose further sanctions if Russia did not change course.
‘World War Three’
On Friday Russian Foreign Minister Sergei Lavrov said the Ukrainian operation against separatists in the east of the country was a bloody crime for which the government in Kiev would face justice.
He repeated the accusation made previously by Moscow that Ukraine was waging a war against its own people.
Soon after Mr Lavrov’s comments, Ukrainian Prime Minister Arseniy Yatsenyuk said Russia wanted to start World War Three by occupying Ukraine militarily and politically and by creating a conflict that would spread to the rest of Europe.
Also on Friday, reports from the port city of Odessa indicated that an explosion at a checkpoint injured at least seven people.
On Thursday morning raids by Ukrainian commandos on pro-Russian checkpoints around the town of Sloviansk left at least two separatists dead.
Also on Friday, reports from the port city of Odessa indicated that an explosion at a checkpoint injured at least seven people.
On Thursday morning raids by Ukrainian commandos on pro-Russian checkpoints around the town of Sloviansk left at least two separatists dead.
- In other developments:
US journalist Simon Ostrovsky, held by pro-Russian separatists in Sloviansk, has been freed and told the Canadian CBC channel that he was beaten up by his captors - Slovakia and Ukraine could sign a deal on gas supplies on Monday, allowing for a reverse flow to reduce Kiev’s dependence on Russia
Unrest began in Ukraine last November over whether the country should look towards Moscow or the West.
Last month Russia annexed Ukraine’s mainly ethnic-Russian Crimea. It followed a referendum in the region that backed joining the Russian Federation but which the West and Kiev deemed illegal.
