"A fire started in Toropets, Tver region, as a result of falling debris from a drone that was repelled by air defense forces," the Tver regional administration said early Wednesday. The oblast declared a "partial evacuation" due to the large blast and surrounding fires. Social media video showed a huge fireball erupt against the night sky.

The above video was likely take from 2km away or more, strongly suggesting this could be the largest explosion of the war on Russian territory. There's widespread speculation that it was a missile and ammo storage site. 

"A NASA satellite image showed a concentrated set of fires east of Toropets, covering an area of 13 square kilometers (5.2 square miles) as of early Wednesday," Amsterdam-based The Moscow Times writes.

"The site corresponds with the location of the Russian Defense Ministry's main missile and artillery directorate arsenal, situated 488 kilometers northeast of Ukraine's border," the report continues. If true, then Moscow is likely to view such a major attack so deep into Russia as having a NATO hand behind it.

Reuters has described it as an "earthquake-sized blast" which also resulted in a mushroom cloud rising high after the explosion. Some online commentators went so far as to say the blast looks suspiciously nuclear.

President Putin last week said it has become clear that NATO countries assist with targeting and that its satellites and coordinating systems must be involved in such attacks.