Russia, the United States, the European Union and Ukraine are due to meet in Geneva to try to reduce escalating tensions over eastern Ukraine.
Deep disagreement over the issue has led to the worst crisis between the US and Russia since the Cold War.
The West accuses Russia of aiding pro-Russian activists who have seized public buildings across the east.
US President Barack Obama has warned Russia against support for further action by armed pro-Russian groups.
"What I have said consistently is that each time Russia takes these kinds of steps that are designed to destabilize Ukraine and violate their sovereignty, that there are going to be consequences," he said.
Meanwhile, Ukraine's military operation against separatists has hit obstacles.
Called an "anti-terrorist" operation by the Kiev government, it started on Tuesday and is designed to dislodge pro-Russia gunmen from local authority buildings in a swathe of cities and towns in eastern Ukraine.
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David Stern BBC News, Donetsk Ukraine's "anti-terrorist" operation is looking more and more a non-event - or worse, an outright fiasco.
Kiev officials have admitted they have no time to lose to extinguish the growing insurrection in the country's east.
But the decision to send the army in has so far backfired terribly.
The soldiers have been helpless and obviously unhappy with being deployed against crowds of civilians.
Ukraine's new leaders are under a great amount of pressure - not just from the Kremlin and the pro-Russian activists, but from their own supporters, outraged at their government's inability to stem the separatist tide.
Right now, everything has been thrown into doubt - even the future of this government and of Ukraine itself.
The biggest question is what will follow.
Pro-Russian activists want referendums on greater autonomy for the south-east or the right to join the Russian Federation.But in several districts, Ukrainian troops met vehement opposition on Wednesday from pro-Russia supporters, who object to the new government in Kiev.
Reports from Mariupol in the south of the Donetsk region say a Ukrainian military unit was attacked with petrol bombs.
Soldiers are reported to have opened fire in response and several people were wounded, including some police.
In the city of Kramatorsk, six military vehicles were commandeered on Wednesday by gunmen, who disarmed the Ukrainian soldiers and sent some of them home on buses.
One Ukrainian officer said he had not "come to fight" and would never obey orders to shoot his "own people".
In another incident, several hundred residents of Pchyolkino, south of Sloviansk, surrounded a column of 14 Ukrainian military vehicles.
Daniel Sandford in Sloviansk gets up close to a Ukrainian armoured vehicle which was "rebranded" with a Russian flagAfter the crowd was reinforced by pro-Russian gunmen, negotiations ensued and the troops were allowed to drive their vehicles away, but only after agreeing to surrender the magazines from their assault rifles.
Ukraine's "anti-terrorist" operation is looking more and more a non-event - or worse, an outright fiasco, reports the BBC's David Stern in Donetsk.
The Geneva meeting is the first time that foreign ministers from the US, the EU, Ukraine and Russia will sit down for talks since the crisis began.
The US and the EU want an end to the occupations in eastern Ukraine and for the estimated 40,000-strong Russian forces massed near the Ukrainian border to pull back.
Troops conducting Kiev's military operation in the east met opposition in several districts 
Pro-Russian gunmen seized Ukrainian military vehicles and took them to Sloviansk 
John Kerry arrives in Geneva where it will be difficult to bridge the gap with his Russian counterpart, Sergei Lavrov A US official, speaking as Secretary of State John Kerry arrived in Geneva, stressed that Russia must "take this opportunity to de-escalate" or face a tightening of sanctions.
Ukrainian Foreign Minister, Andriy Deshchytsya, called on Russia "not to support terrorist activities in eastern Ukraine."
As if to further illustrate the gulf between the West and Russia over the crisis, Russia's foreign ministry accused Washington of "the persistent unwillingness or inability to see reality as it is in fact, and in a striving to impose on the rest of the world a distorted perception of what is happening in southeast Ukraine."
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Jonathan Marcus BBC diplomatic correspondent Nato's announcement is about reassuring worried allies in the Baltic republics and central Europe, as well as signalling to Moscow that when it comes to the alliance's core business - the defence of its members' territory - Nato is as vigilant as ever.
Air patrols over the Baltic republics are to be stepped up; Nato warships will deploy to the eastern Mediterranean and the Baltic Sea.
And on land, staff officers will be dispatched to oversee an enhanced programme of exercises and preparedness in the countries most concerned.
Nato will also review and reinforce its defence plans. In the longer term additional deployments may well be made. Nato sources indicate that this is only the first step of several that could be taken if the relationship with Russia sours further.
Russia, which strongly opposed the ousting of Ukraine's pro-Moscow President Viktor Yanukovych in February, has proposed a new constitution which devolves more power to the regions.Expectations for the talks are low, says the BBC's Gavin Hewitt in Geneva.
NatoRussia's stance over eastern Ukraine and its annexation of Crimea in March continue to cause concern in Nato member countries with large Russian-speaking minorities, such as Latvia and Estonia.
So Nato announced on Wednesday that it was beefing up its eastern members' defences.
In Brussels, Nato Secretary General Anders Fogh Rasmussen promised "more planes in the air, more ships on the water, more readiness on the land".
He called on Russia to make clear it did not "support the violent actions of well-armed militias or pro-Russian separatists in eastern Ukraine".
Are you in eastern Ukraine? What is the situation like where you are? You can email your experiences to haveyoursay@bbc.co.uk using the subject line 'Eastern Ukraine'.