Nato leaders have ramped up the pressure on President Vladimir Putin - with a warning they will fight back against any incursion by Moscow against alliance members on Russia's borders.
Ahead of a two-day summit meeting in Wales, US president Barack Obama made clear the alliance stood ready to defend the territorial integrity of "every single one" of its 28 member states.
Meanwhile French president Francois Hollande bowed to months of pressure from fellow allies with the announcement that he was suspending the delivery of the first of two giant Mistral class helicopters destined for the Russian navy.
Prime Minister David Cameron said the summit would provide an opportunity for Nato to reaffirm its commitment to collective security as he condemned Russia's "appalling actions" in eastern Ukraine.
"In this dangerous and difficult world, Nato has an absolutely key role in providing our collective security and that's what the next two days are going to be all about," he said.
Mr Putin showed signs he was feeling the pressure, issuing a call to the pro-Russian separatists to "stop advancing" while urging the Ukrainian army to withdraw its troops from the region.
However his words are unlikely to satisfy Western leaders who have threatened to tighten the economic sanctions against Russia unless it withdraws its support for the insurgents and ends its attempts to destabilise the government in Kiev.
They came after a morning of confusion which saw Ukrainian president Petro Poroshenko announce he had agreed a ceasefire only for the rebels to repudiate it while Moscow said that it was not part of the conflict.
The strongest intervention came from Mr Obama during a symbolic visit to Estonia - one of the three Baltic states bordering the Russia which were once part of the Soviet Union and which fear they could be the next target of Mr Putin's aggression.
Unlike Ukraine - which only has a partnership agreement with Nato - Estonia, Latvian and Lithuania are full alliance members and Mr Obama made clear the alliance would honour its obligation to defend them in the event of an attack.
"You lost your independence once before. With Nato, you'll never lose it again," he declared before a packed audience in a concert hall in the Estonian capital, Tallinn.
"We will defend our Nato allies - every ally. In this alliance, there are no old members or new members, no senior partners or junior partners - there are just allies, pure and simple. And we will defend the territorial integrity of every single one."
Earlier he announced that he would be sending further air force units to the region in fresh show of US military strength.
Nato leaders meeting in Wales are expected to agree to the formation of a new high-readiness "spearhead" force able to deploy "several thousand" troops - backed by naval and air support - within a matter of days in response to a crisis.
The alliance is also planning to establish forward operating bases in eastern Europe where fuel, ammunition and other supplies can by stockpiled ready for use in operations.
With Britain and the US two of only four member states to meet the alliance's obligation to spend 2% of GDP on defence, Mr Cameron and Mr Obama will be pressing other leaders to raise their spending on their armed forces.
The US president had some harsh words for European members who had failed to deliver on their commitments in the past.
"For I think a certain period of time there was a complacency here in Europe about the demands that were required to make sure that Nato was able to function effectively," he said.
"Obviously what's happened in Ukraine is tragic, but I do think it gives us an opportunity to look with fresh eyes and understand what it is that's necessary to make sure that our Nato commitments are met."
Mr Cameron and Mr Obama will hold talks tomorrow with president Poroshenko - together with Mr Hollande, German chancellor Angela Merkel and Italian prime minister Matteo Renzi - ahead of the main summit meeting.
"The alliance must demonstrate clearly that Russia's actions are unacceptable and that we stand by the people of Ukraine and their right to decide the future of their country," a UK government source said.
"The meeting will provide leaders with the opportunity to hear president Poroshenko's assessment of the latest situation on the ground and his discussions with president Putin. It will also send a clear signal of their support for Ukraine's sovereignty and that the onus is on Russia to de-escalate the situation."