Portuguese Air Force fighter F-16 (L) and Canadian Air Force fighter CF-18 Hornet patrol over Baltics air space, from the Zokniai air base near Siauliai November 20, 2014. REUTERS/Ints Kalnins

By David Mardiste and Jonathan Saul

AMARI Estonia/LONDON (Reuters) - NATO warplanes have had to scramble 400 times this year in response to an increase in Russian air activity around Europe not seen since the Cold War, the alliance's chief 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 a secessionist movement in the eastern part of Ukraine.

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

He said the flights - which have risen 50 percent over the last year - posed a risk to commercial air traffic.

"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."

Vice Admiral Peter Hudson, commander of NATO's Allied Maritime Command, said Russia was using its military forces in a "much more assertive manner", which included more Russian naval activity in the Baltic region.

"From a NATO perspective, we are going about our legitimate business in international waters working with our allies. They are occasionally interfering, occasionally operating as a nuisance," Hudson told Reuters in London.

"Safety has not been breached, but it is just a style of behaviour which we have not seen for 25 years, since the end of the Cold War," he said.

Stoltenberg said most of the Russian flights were close to NATO air space, with "very limited numbers of violations".

NATO allies remain concerned that France could still decide to sell an advanced military helicopter carrier to Russia, Stoltenberg said, but added that this was not a decision that NATO had any authority over.

(Editing by Sonya Hepinstall and Hugh Lawson)