Ukrainian troops are stationed on the border with Russia, in smaller numbers. Photo:BBC

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 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.

Moscow formally annexed Crimea after the predominantly ethic-Russian region held a referendum which backed joining the Russian Federation.

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 that said Moscow did not have any "expansionist views" and that "nobody should fear Russia".

On Sunday, Ukraine's National Security and Defence Council chief Andriy Parubiy told a 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."

 

The Russian Ambassador to the EU, Vladimir Chizhov, has said that Russia does not have any "expansionist views."

'Adversary'

The comments of 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."

A war memorial in Tiraspol, Trans-Dniester. Nato fears Russian troops could move there quickly

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 the Ukrainian 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.

On Sunday, 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."

Earlier, Russia's ambassador to the EU Vladimir Chizhov defended Russia's actions in Crimea and warned the US against sending troops or military aid to Ukraine, saying it would be a "grave mistake".

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It was quick, well-organised and over before the Ukrainians could fight back; much like Russia's entire takeover of Crimea”

Mark LowenBBC News, Novofedorivka base

• Crimea base falls

He told the BBC that Russia did not recognise the new government in Kiev but he expected it to safeguard the rights of ethnic Russians in Ukraine.

"We want to see the interests of people living in eastern Ukraine - ethnic Russians, Russian speakers, all the population of eastern Ukraine - to be taken into account by whoever is the authority in Kiev today" he said.

He added that Russia did not have any intention of sending troops into other parts of Ukraine or Europe.

Bases stormed

Correspondents 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 on Saturday.

The BBC's Mark Lowen also witnessed the takeover of the Novofedorivka base in eastern Crimea by Russian troops.

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 acted to protect its "compatriots" in Crimea from "fascists" moving in from the mainland Ukraine.

Ukraine and the West have condemned the referendum as "illegal".

The US and EU have responded with a series of sanctions targeting those individuals - including senior officials - involved in what Washington and Brussels describe as Crimea's "annexation".