MOSCOW, March 20 (Reuters) - Ukraine struck a major Russian strategic bomber airfield on Thursday with drones, triggering a huge blast and fire about 700 km (435 miles) from the front lines of the war, Russian and Ukrainian officials said. Videos verified by Reuters showed a huge blast spreading out from the airfield, wrecking nearby cottages. Russia's defence ministry said air defences had shot down 132 Ukrainian drones over Russian regions. The base in Engels, which dates back to Soviet times, hosts Russia's Tupolev Tu-160 nuclear-capable heavy strategic bombers, known unofficially as White Swans. Roman Busargin, governor of Saratov, said...