Stoltenberg, on a visit to Norway, also said that while Russia's behaviour had grown more aggressive, it did not pose an immediate threat to NATO.

"What we have seen in the past 24 hours in Ukraine is a reminder that the truce is very fragile, that there are still violations, and that it is increasingly important to support all the efforts, all the initiatives, to respect the ceasefire," he told reporters.

A surge in violence in eastern Ukraine has left at least 26 people dead, most of them separatists, since Wednesday, according to Ukrainian authorities and the rebels, despite the February peace accord.

Kiev and the West accuse Russia -- which annexed the Crimean peninsula in March 2014 -- of arming the rebellion in eastern Ukraine and of sending in regular troops, claims Moscow fiercely denies.

"Russia has a special responsibility because Russia supports the separatists, because Russia provides them with heavy arms, and because Russia also has troops in eastern Ukraine," the NATO chief said.

Stoltenberg, on his first official visit to his native country since taking over as NATO secretary general last year, again called on all parties to respect the truce deal.

Despite the flare-up of tensions and subsequent concerns among some NATO members -- in particular the Baltic states and Poland -- "we do not see an immediate threat from the East against any NATO country," Stoltenberg also told Norwegian radio NRK.

"What we see is increased unpredictability, increased insecurity, increased nervousness," the former Norwegian prime minister said.

He painted a picture of a heavily re-militarised Russia "which is unfortunately now more aggressive than it was a few years ago," and which doesn't hesitate "to resort to military force to redraw the borders in Europe," citing Crimea, Ukraine and Georgia as examples.