By Andrew Osborn LONDON (Reuters) - Britain summoned the Russian ambassador on Thursday and asked him to explain why two Russian "Bear" long-range bombers had flown over the English Channel the previous day, forcing British authorities to reroute civil aircraft. A British government source told Reuters the incident, which forced Britain to scramble Typhoon interceptor jets, was viewed as "a significant escalation" and marked a change in strategy since Russian aircraft had previously largely confined themselves to flying close to Scotland. Civil aircraft flying to the UK had to be rerouted," the source said. They haven't flown this far south before." The Foreign Office said it had summoned Russian Ambassador Alexander Yakovenko to account for the incident, saying the episode was part of an increasing pattern of "out of area operations" by Russian aircraft.