London (AFP) - Russia could inflict the tactics it is accused of using to destabilise Ukraine on Baltic members of NATO, Britain's defence minister warned in comments to British newspapers.

Michael Fallon said that NATO must be prepared for Russian aggression in any form, and warned of a "real and present danger" to Baltic members of NATO -- eurozone members Estonia, Latvia and Lithuania.

It comes after Kiev called for UN peacekeepers to help implement a tattered ceasefire between Ukrainian forces battling the advance of pro-Russian rebels in the east of the country.

"NATO has to be ready for any kind of aggression from Russia, whatever form it takes. NATO is getting ready," Fallon said in comments on Wednesday to journalists accompanying him on a trip to Sierra Leone.

Kiev and the West accuse Russia of arming the rebels and sending thousands of soldiers into Ukraine, charges denied by Russian President Vladimir Putin.

"I'm worried about Putin," Fallon said according to newspaper The Times. "I'm worried about his pressure on the Baltics, the way he is testing NATO."

In comments also reported by the Daily Telegraph, Fallon described as a "very real and present danger" the possibility Russia could repeat towards Baltic states the covert warfare techniques it is accused of using in Ukraine.

British Prime Minister David Cameron on Wednesday said "effectively one country is challenging the territorial integrity of another country" to describe the situation in Ukraine.

"Those Russian-backed rebels in eastern Ukraine, they are using Russian rocket launchers, Russian tanks, Russian artillery, you can't buy this equipment on eBay...it's come from Russia," Cameron said.

"So we have to be very firm and strong about the sanctions and say to Vladimir Putin: 'What you are doing is unacceptable and it will have economic and financial consequences for many years to come if you do not desist with your behaviour'."

The defence minister said the situation was not a new Cold War, because it was already "pretty warm", according to the Times.

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Fallon referred to the interception of Russian bombers flying close to British airspace, and Russia's detention of an Estonian policeman that sent tensions soaring between Moscow and Tallinn.

"You have tanks and armour rolling across the Ukrainian border and you have an Estonian border guard being captured and not yet still returned," Fallon reportedly said.

"When you have jets being flown up the English Channel, when you have submarines in the North Sea, it looks to me like it's warming up."

Asked for comment, a spokesperson for the Defence Ministry confirmed that Fallon had spoken to journalists but did not confirm his exact words to AFP.

His comments came after hundreds of exhausted Ukrainian troops retreated from the strategic railway hub of Debaltseve following a fierce assault by pro-Russian rebels on Wednesday.