KYIV, Dec 26 (Reuters) - A drone believed to be Ukrainian penetrated hundreds of kilometers through Russian airspace, causing a deadly explosion at the main base for Moscow's strategic bombers in the latest attack to expose gaps in its air defences.

Moscow on Monday said it had shot the drone down causing it to crash at the Engels air base, where three service members were killed. Under its usual policy on incidents inside Russia, Ukraine did not comment.

The base, the main airfield for the bombers that Kyiv says Moscow has used in recent months to attack Ukrainian civilian infrastructure, is hundreds of miles from the Ukrainian frontier. The same planes are also designed to launch nuclear-capable missiles as part of Russia's

The base, the main airfield for the bombers that Kyiv says Moscow has used in recent months to attack Ukrainian civilian infrastructure, is hundreds of miles from the Ukrainian frontier. The same planes are also designed to launch nuclear-capable missiles as part of Russia's long-term strategic deterrent