America has reassured its East European allies of Nato's support as the worst post-Cold War stand-off between Russia and the West continues.
US Vice President Joe Biden phoned Czech Prime Minister Bohuslav Sobotka and Hungary's Prime Minister Viktor Orban and "underscored the ironclad commitment of the US" to the collective defence of Nato members.
They "agreed that Russia would face further costs for its destabilising actions in Ukraine and massing of troops along the Russian-Ukrainian border," the White House said.
The US has ruled out sending American or Nato forces into Ukraine, but has begun deploying 600 US troops to bolster defences in the nearby countries of Poland, Lithuania, Latvia and Estonia.
Meanwhile, Western leaders are urging Russia to help free a team of international military observers in eastern Ukraine.
Eight members of the Organisation for Security and Co-operation in Europe (OSCE) were travelling with five Ukrainian troops when they were detained by separatists in the flashpoint town of Slavyansk.
Colonel Axel Schneider says his team are being well-treated Team leader Colonel Axel Schneider appeared before cameras in Slavyansk to say they are being well-treated and "haven't been touched" by their kidnappers.
They are being protected by the town's self-appointed mayor Vyacheslav Ponomaryovt and "treated as his guests," he said.
"We have no indication when we will be sent home to our countries," he said.
"We wish from the bottom of our hearts to go back to our nations as soon and as quickly as possible."
On Saturday, Moscow said it would take "all possible steps" to free the OSCE monitors as their capture was condemned by Western leaders.
However, rebels accused the observers of being "Nato spies" and vowed to continue holding them.
A barricade around the state security service building in Slavyansk It comes as Britain and other G7 nations agreed new sanctions against Russia.
Speaking at a news conference in Malaysia, US President Barack Obama said it was important for the world to send Russia a message.
"We're going to be in a stronger position to deter Mr Putin when he sees that the world is unified and the United States and Europe is unified rather than this is just a US-Russian conflict," he said.
He said Russia had not "lifted a finger" to get pro-Russian separatists in Ukraine to comply with an international agreement to defuse the crisis.
"In fact, there's strong evidence that they've been encouraging the activities in eastern and southern Ukraine," he said.
Senior EU diplomats are to hold an emergency meeting in Brussels on Monday to discuss sanctions against Moscow.
British Foreign Secretary William Hague said the EU and US would be bringing forward further travel bans and asset freezes against Moscow over coming days as punishment for its "bullying" behaviour.
Speaking on Sky's Murnaghan programme, he said damage to the UK economy from imposing trade sanctions on Russia would be a "price worth paying".
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