Russian military aircraft are posing a threat to civilian planes by turning off communications devices and failing to file flight plans, the US ambassador to Nato said. Russia has stepped up flights near Nato's borders last year, the alliance says, coinciding with increased East-West tension over Russia's role in the Ukraine crisis. Nato aircraft have scrambled more than 400 times this year to intercept Russian aircraft, up 50 per cent from the 2013 total. US Ambassador Douglas Lute said on Monday there had been "multiple incidents" where Russian military aircraft had not filed flight plans or spoken to civilian air traffic controllers and had turned off transponders that send information about the plane. This made the planes virtually invisible to air traffic controllers, he said during a news conference. "These Russian actions are irresponsible, pose a threat to civilian aviation and demonstrate that Russia is flagrantly violating international norms," he said. Nato's secretary general, Jens Stoltenberg, urged Russia to conduct the flights "in a more responsible way". Britain also voiced concern about Russian probing of its air space. In late October, Nato said its aircraft tracked Russian strategic bombers over the Atlantic and Black Sea and sorties of fighters over the Baltic. At sea, too, Russia has been more active, leading one Nato commander to draw comparisons with cold war behaviour. A squadron of Russian warships entered the English Channel on Friday. Increased Russian assertiveness in the air and at sea has raised concern in Nato about the risks of miscalculation. Military personnel from the 28-nation alliance and Russia are no longer so familiar with procedures and behaviour that were standard during the cold war, raising the risk of an accidental incident, one diplomat said. A report last month by the European Leadership Network, a security think tank, cited an increase in incidents involving Russian and Western militaries, raising the risk of accidental military confrontation. It said there had been a "close encounter" in March between a passenger plane taking off from Copenhagen and a Russian reconnaissance aircraft. Douglas Barrie, an aerospace expert at the International Institute for Strategic Studies, a London think tank, said the higher Russian military profile partly reflected better funding after years in which a cash-starved air force did little because of inadequate training, fuel shortages and poor maintenance.