NATO planes have been scrambled 400 times this year as they respond to a 50% rise in Russian military air activity around Europe compared with 2013.

They include a number of high-profile incidents where RAF jets have intercepted Russian aircraft approaching UK airspace.

Moscow was also suspected of being behind a mysterious vessel, thought to be a submarine, which was spotted in waters off Sweden earlier this year.

The alliance said the activity was at a level not seen since the Cold War, and Russia was using its military in a "much more assertive manner", which included more naval cases in the Baltic region.

NATO has been trying to boost its land, air and sea defences since Russia annexed Crimea and backed separatists in eastern Ukraine.

More than 1,000 UK military personnel have joined a training exercise in Poland in a show of NATO strength.

The commitment, Britain's largest to the region since 2008, is an attempt to support allies in Eastern Europe and the Baltic States as part of a planned series of NATO manoeuvres throughout the autumn.

The NATO chief said the alliance "remains vigilant" and was "ready to defend all our allies against any threats".

Secretary-General Jens Stoltenberg said the problem was not just where the Russians are flying but they are not turning on transponders or communicating.

He said the flights posed a risk to commercial air traffic, saying: "This pattern is risky and unjustified."

Mr Stoltenbeg said: "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."

And he said most of the Russian flights were close to NATO airspace, with "very limited numbers of violations".

Vice Admiral Peter Hudson, commander of NATO's Allied Maritime Command, said: "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."