Mr Stoltenberg declines to put a figure on the scale of the Russian deployment, but he is struck by the presence of sophisticated air defences inside the rebel-held enclave in Ukraine. We see a lot of advanced, for instance, air defence systems. This has to come from somewhere and it has to be operated by people who are able to operate this kind of advanced military equipment, he says.

As for who broke the Minsk Agreement, Mr Stoltenberg places the blame firmly on Russia. The developments are going in the wrong direction, he says. The idea with the Minsk Agreement was that Russia should withdraw its forces. But instead of withdrawing them, they are sending in new forces and thats undermining the whole ceasefire and the Minsk Agreement.

So has Russia invaded Ukraine? Mr Stoltenberg, for all his candour, still declines to endorse such a blunt term. There are Russian forces inside Ukraine and there has been a military build-up and thats in violation of the Minsk Agreement, he replies. Its a threat to the ceasefire - and thats the reason why were calling on Russia to withdraw its forces.

What might be the rationale for this Russian build-up? One plausible explanation is that Mr Putin and the rebels are planning a joint offensive to enlarge the pro-Russian statelet by capturing more Ukrainian territory, possibly beginning with the port city of Mariupol.

Ill be very careful on speculating on intentions, says Mr Stoltenberg. But I see what they do: Nato acts based on what we see Russia actually doing. And what they are doing is violating international law, they are violating the whole rules-based system of working together in Europe - which was developed over decades - and they are also using force to change borders in Europe.

In addition, Russia is sending more and more jet bombers and fighters to probe the air defences of Europe. In itself, this is nothing new: exercises of this kind have always happened.

But Mr Stoltenberg points out that Nato fighters have intercepted Russian intruders over 100 times so far this year a threefold increase in the number of incidents recorded in 2013. Moreover, the Russian planes often ignore safety procedures, raising the risk of a catastrophic collision with civil airliners in European airspace.

The problem is that many of the Russian pilots dont turn on their transponders, they dont file their flight plans and they dont communicate with civilian air traffic control, says Mr Stoltenberg.

This poses a risk to civilian air traffic and therefore this is a problem, especially when the Russian activity increases.

In response to Russias new challenge, Nato has taken a series of decisions. Five times as many fighter aircraft have been deployed to protect its Baltic members Estonia, Latvia and Lithuania and more warships have been dispatched to the Black Sea. Natos armies are creating a spearhead force designed to guard the alliances most exposed members.

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Jens Stoltenberg: the new man at Nato who must stand up to Putin and his tanks