NATO chief Jens Stoltenberg said Thursday that the transatlantic alliance remained vigilant after an increase in Russian military manoeuvres in European airspace.

"NATO is strong, it remains vigilant," he said during a visit to Athens.

Russian strategic bombers, fighters and tanker aircraft have been intercepted on Tuesday and Wednesday in what NATO said earlier was "unusual" activity.

"This strength is our answer" to what NATO has termed large-scale Russian military manoeuvres over the Atlantic, Baltic, North and Black seas earlier this week, when they skirted NATO airspace.

"We are not in a Cold War situation, but Russia has undermined a lot of trust," he said. "We must keep NATO strong."

NATO allies sent up aircraft to intercept and identify them, the military bloc said, without mentioning the Ukraine crisis or the heightened tensions over the conflict.

NATO's eastern members, some of whom like the Baltic states were once ruled from Moscow, have been particularly nervous, and the US-led alliance has put in place additional aircraft and personnel on a rotating basis to reassure them.