Nato: Russia unlikely to invade Ukraine

"Now I think probably the most likely course of action is that he will continue doing what he's doing – discrediting the government, creating unrest, trying to set the stage for a separatist movement."

He noted that Russian special forces are probably operating in east and south Ukraine, however. "Remember that Putin denied their presence and now he has admitted to their presence in Crimea. The same thing will come out of Ukraine as time rolls out."

With the US and the EU saying they will not impose economic sanctions on Russia unless there is a full-scale invasion, Breedlove added: "In that case, I think it's the most troublesome for Nato because if the forces do not come across the border, my guess is that many will want to try to quickly go back to business as usual."

He also told press in a separate meeting with Canadian Prime Minister Stephen Harper that "Russia's aggression in Ukraine has caused a paradigm shift".

"If there was any doubt of the relevance of Nato and the strength of the trans-Atlantic bond before now, the last few weeks have cleared that up and reinforced the need for our essential core tasks," he added.

"What we have to do is build forces that reassure our allies, but are not necessarily provocative to the Russians."

Harper, for his part, described events in Ukraine as "a slow-motion invasion on the part of the Putin regime".

The remarks came amid more fighting in east Ukraine on Monday, where government forces are trying to retake rebel positions.

Ukrainian authorities say insurgents shot down a fourth military helicopter in the Sloviansk region and that six people were killed in clashes with a group of 800 pro-Russia separatists armed with heavy-calibre guns and mortars.

Russian media say some 20 rebels were killed.

The Russian foreign ministry urged Ukrainian authorities to "to their senses, stop the bloodshed, withdraw forces, and finally sit down at the negotiating table".

It also published a file describing the February revolution in Ukraine as a "coercive rebellion" carried out "under the influence of extremists from ultranationalistic and neo-Nazi forces, and with the active multidimensional support of the USA and the European Union and its members".

Elsewhere in the region, Russia cancelled an agreement with Lithuania under which the former Soviet state could inspect military facilities in Russia's Kaliningrad exclave.

Meanwhile, Moldova put its troops on alert amid concerns that Russian forces or pro-Russian separatists in the breakaway Transniestria region might stir up trouble.

The deterioration has prompted Germany and Lithuania to issue a travel alert for Crimea and south and east Ukraine more broadly.

Their advice for people to stay away comes after the UK and US published similar warnings in late April.