Nato warns of Russian army build-up on Ukraine border
Ukrainian troops are stationed on the border with Russia, in smaller numbers Nato's military commander in Europe has issued a warning about the build-up of Russian forces on Ukraine's border.
Supreme Allied Commander Europe Gen Philip Breedlove said Nato was in particular concerned about the threat to Moldova's Trans-Dniester region.
Russia said its forces east of Ukraine complied with international agreements.
The build-up has been allied with Russia's annexation of Crimea from Ukraine, following the removal of Ukraine's pro-Moscow president.
Ukrainian Foreign Minister Andriy Deshchytsia warned that the risk of war with Russia was growing.
"The problem with [Russian President Vladimir] Putin is that he doesn't want to talk to - not only to the Ukrainian government - but also to the Western leaders," Mr Deshchytsia told the BBC.
The Russian ambassador to the EU, Vladimir Chizhov, has said that Russia did not have any "expansionist views"
"And this is quite a danger for the decision-making process. We could only expect that he might invade."
Meanwhile, US Deputy National Security Adviser Tony Blinken said Washington was reviewing every request Ukraine was making for help.
"When it comes to military assistance, we're looking at it," he told CNN.
But he added: "The facts are these: even if assistance were to go to Ukraine, that is very unlikely to change Russia's calculus or prevent any invasion."
President Barack Obama earlier ruled out sending US troops to Ukraine.
Moscow formally annexed Crimea after the predominantly ethnic-Russian region held a referendum which backed joining Russia.
Kiev and the West have condemned the vote as "illegal".
A war memorial in Tiraspol, Trans-Dniester. Nato fears Russian troops could move there quickly Russian flags have now been hoisted at 189 Ukrainian military units and facilities in Crimea, the Interfax news agency reports.
Moscow's ambassador to the EU told the BBC the "reunification" had not been pre-planned but was the end of an "abnormality" which had lasted for 60 years.
Vladimir Chizhov also said said Moscow did not have any "expansionist views" and that "nobody should fear Russia".
But he warned the US against sending troops or military aid to Ukraine, saying it would be a "grave mistake".
Also on Sunday, Ukraine's National Security and Defence Council chief Andriy Parubiy told a big rally in Kiev: "The aim of [President Vladimir] Putin is not Crimea, but all of Ukraine... His troops massed at the border are ready to attack at any moment."
'Adversary'The comments by Gen Breedlove came at an event held by the German Marshall Fund think-tank in Brussels.
He said: "The [Russian] force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready."
He added: "There is absolutely sufficient force postured on the eastern border of Ukraine to run to Trans-Dniester if the decision was made to do that and that is very worrisome.
"Russia is acting much more like an adversary than a partner."
Trans-Dniester is a narrow strip of land between the Dniester river and Ukraine's south-western border and it proclaimed independence from Moldova in 1990.
The international community has not recognised its self-declared statehood.
As Crimea was annexed, the Trans-Dniester Supreme Soviet sent a request asking to join the Russian Federation.
Meanwhile, Russia's Deputy Defence Minister Anatoly Antonov told the Itar-Tass agency: "The Russian Defence Ministry is in compliance with all international agreements limiting the number of troops in the border areas with Ukraine."
Bases stormedCorrespondents say Russian forces appear to be stepping up their efforts to secure full military control of all of Crimea.
The BBC's Ian Pannell, in Belbek, says the few remaining Ukrainian troops on the peninsula feel beleaguered and abandoned by their commanders.
He saw Russian troops use stun grenades and automatic weapons in a raid on the Belbek airbase, near Sevastopol, on Saturday.
The BBC's Mark Lowen also witnessed the takeover of the Novofedorivka base in western Crimea by Russian troops.
A big unity rally is being staged in Ukraine's capital Kiev 
Pro-Russian men beat an unknown man during the takeover of the Belbek airbase on Saturday 
Ukrainian soldiers in Belbek were given an ultimatum to leave the base on Saturday. Russian soldiers and pro-Russian protesters stormed the base and forced Ukrainian troops to leave.
Russia annexed Crimea following a referendum on 16 March, which came after the overthrow of Ukrainian President Viktor Yanukovych in February.
The Kremlin said it had acted to protect its "compatriots" in Crimea from "fascists" moving in from the mainland Ukraine.
The US and EU have responded with a series of sanctions targeting those individuals - including senior officials - involved in Crimea's annexation.
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