Russia Wants World War Three, Says Ukraine
Ukrainian Prime Minister Arseniy Yatsenyuk has accused Russia of wanting to start "a third world war".
Mr Yatseniuk said that Russia wanted to occupy Ukraine "militarily and politically", creating a conflict that would spread to the rest of Europe.
"Attempts at military conflict in Ukraine will lead to a military conflict in Europe," Mr Yatseniuk told the interim cabinet in remarks broadcast live.
"The world has not yet forgotten World War Two, but Russia already wants to start World War Three."
US President Barack Obama said he would consult key European leaders later on the possibility of imposing new sanctions against Russia.
Speaking in Seoul, Mr Obama said he wanted to coordinate a united approach with Western US allies.
The president spoke after US Secretary of State John Kerry accused Russia of "deception" and "destabilisation" in Ukraine and hinted at more sanctions.
Speaking at the State Department in Washington, Mr Kerry warned Moscow time was running out for it to change course.
"Russian President Vladimir Putin and Russia face a choice," he said in unusually blunt language.
"If Russia chooses the path of de-escalation ... all of us will welcome it. But if Russia does not, the world will make sure that the costs for Russia will only grow."
Mr Kerry added this would be a "grave" and "expensive mistake".
He also said the Kremlin has not honoured an agreement aimed at defusing the crisis, echoing earlier comments by US President Barack Obama.
The Geneva accord between Russia, Ukraine, the US and EU compelled armed groups to put down their weapons and vacate official buildings.
The US says Moscow is stoking unrest and separatist sentiment in the east of Ukraine after its annexation of Crimea.
Russia accuses Washington of encouraging a pro-Western government to adopt anti-Russian policies.
The crisis in Ukraine is becoming increasingly more violent as separatists take control of public buildings and roads. The government has launched a campaign to remove them.
On Thursday, Ukraine's interior ministry said up to five "terrorists" had been killed during an operation to clear checkpoints in the eastern town of Slavyansk.
Russia announced military exercises involving ground and air forces near its border with Ukraine in response to the operation. Mr Putin has said there will be "consequences" if Kiev has used its army against the activists.
UN chief Ban Ki-moon has warned the crisis is threatening to "spin out of control" and urged all sides to "refrain from violence".