Russia could repeat the tactics it has used to destabilise Ukraine in the Baltics countries, British Defence Minister Michael Fallon warned in comments made to British newspapers on Wednesday.

He said that NATO must be prepared for Russian aggression in any form, and warned of a "real and present danger" to the Baltic countries.

"NATO has to be ready for any kind of aggression from Russia, whatever form it takes. NATO is getting ready," Fallon said. "I'm worried about Putin. I'm worried about his pressure on the Baltics, the way he is testing NATO," The Times reported Fallon as saying.

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

On Wednesday, British Prime Minister David Cameron said, talking about the situation in Ukraine, that "effectively one country is challenging the territorial integrity of another country ... 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 behavior'," he continued.

Fallon rejected the description of the situation as a new Cold War, pointing out that it is already "pretty warm", according to The Times.

Fallon went on to talk about the interception of Russian bombers close to British airspace, and Russia's detention of Eston Kohver, an Estonian security official - an event 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.

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.