Nato chief warns Russia against 'green men' tactics

He added: “I want to clarify one thing: If foreign forces seep into Nato territory, and if we can demonstrate that this approach is an aggression - then this means Article V [of the Nato treaty on collective defence] … this means a military response to the actions of the aggressor”.

Breedlove and Nato secretary general Anders Fogh Rasmussen sent out a similar message in an op-ed in the Wall Street Journal on Saturday.

“Today and in the future, Nato means one for all, all for one”, they said.

They noted that when the alliance holds its summit in Wales in September, it will “upgrade” its rapid-reaction force to deal with potential ‘green men’ scenarios.

They said the force will “deploy at the first sign of trouble … travel light but strike hard if needed”.

“Having the right capabilities, in the right place, at the right time can make the difference between threat and reassurance, between war and peace … these measures are necessary to adapt to a dangerous world and to respond to Russia's double-game”.

Estonia and Latvia are considered the most vulnerable Nato members because, like Crimea, they host large ethnic Russian minorities.

Breedlove spoke out in German media the same day German foreign minister Frank-Walter Steinmeier met with his French, Russian, and Ukrainian counterparts for a five-hour long fireside talk at the Villa Borsig in Berlin.

The talks come amid new photo evidence that Russia is sending Russian-flagged military vehicles into east Ukraine on a daily basis.

They also come amid concern Russia will try to use humanitarian aid to reinforce pro-Russia rebels, who have suffered setbacks, in what some analysts are calling “beige men” tactics by reference to the Russian aid personnel's beige clothes.

Steinmeier said the Berlin meeting was “difficult” but “made progress on some points” and that the FMs are likely to meet again this week.

Ukraine’s Pavlo Klimkin said they would probably have to hold “many more” five-hour long conversations before there is progress.

EU foreign ministers also discussed Ukraine at an extraordinary meeting in Brussels on Friday.

They urged Russia to allow international monitors from the OSCE, a multilateral European body in which Russia holds a veto, to work on the Russia-Ukraine border.

They also urged Russia to “reach an agreement as soon as possible” on Ukraine gas prices, after European Commission chief Jose Manuel Barroso said he is to hold new gas supply talks with Russian leader Vladimir Putin and Ukrainian President Petro Poroshenko.

The gas initiative comes amid concern that eastern EU states who get Russian gas via Ukraine might suffer cut-offs in winter.

Slovakia, which gets 83 percent of its gas from Russia, and Hungary, which gets 80 percent, spoke out against EU sanctions on Russia last week.

Slovak PM Robert Fico said on Thursday: “Why should we jeopardise the EU economy? … If there is a crisis situation, it should be solved by other means than meaningless sanctions”.

Hungary’s Viktor Orban told Hungarian media on Friday “the sanctions policy pursued by the West … causes more harm to us than to Russia”. He added: "In politics, this is called shooting oneself in the foot”.

For his part, Lithuanian FM Linas Linkevicius, whose country gets 100 percent of its gas from Russia, noted the same day: “I would say: Better shoot yourself in the foot, than let yourself be shot in the head”.