AMARI, Estonia (Reuters) - Nato war planes have had to scramble 400 times this year in response to an increase in Russian air activity around Europe, a rise of 50 per cent over last year, the new secretary-general Jens Stoltenberg, said on Thursday.

North Atlantic Treaty Organisation members have sought to fill gaps in the alliance's land, air and sea defences since Russia annexed Crimea and backed the secessionist movement in the eastern part of Ukraine.

Speaking to US, German and Estonian troops at a newly expanded and renovated air base in western Estonia, which once housed a Soviet military installation, Stoltenberg said the problem was not just where the Russians are flying but that they are not turning on their transponders or communicating.

As such, the flights pose a risk to commercial air traffic, he said. "This pattern is risky and unjustified, so Nato remains vigilant. We are here and we are ready to defend all our allies against any threats," Stoltenberg said in a speech after meeting Estonian Prime Minister Taavi Roivas.

"It is a pattern which we have not seen for many years and it is a pattern that reminds us of the way they conducted these kind of military air activities back in the time of the Cold War."

Most of the flights are close to Nato airspace, with "very limited numbers of violations", he said.

Nato allies remain concerned that France could still decide to sell an advanced military helicopter carrier to Russia, Stoltenberg said, but noted it was not a decision that Nato had any authority over. "Nato does not possess or Nato doesn't sell military equipment, that is something the nations are doing or not doing," he said.