An overnight Ukrainian drone attack triggered a massive explosion and large fires in northwestern Russia's Tver region. Is believed the initial drone strike hit an important Russian weapons stockpile.

"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.

Kyiv owned up to the attack, with a government statement saying Ukraine's Security Service and Intelligence and Special Operations Forces launched the strike. Russian state media has been quiet about the details or confirmation, and the Russian defense ministry has seemed slow to weigh in. This is likely because it was a highly sensitive facility and this is a huge blow to Russia's military. 

A EuroNews/AP report has quoted a Ukrainian source as saying "the depot housed Iskander and Tochka-U missiles, as well as glide bombs and artillery shells."

The big overnight cross-border drone attack came amid a broader wave that included possibly hundreds of UAVs. Russia's defense ministry said its air defense forces shot down 54 of them over five regions during the night morning hours attack.

The drone assault on the arsenal triggered secondary blasts resulting in fires that spread across a huge expanse:

According to a review summary of what's now about the Tver site that exploded via Al Jazeera:

• According to a RIA state news agency report from 2018, Russia was building an arsenal for the storage of missiles, ammunition and explosives in Toropets.
• The arsenal was built in 2015 at a cost of 3.6 billion roubles ($39m), according to Rossiyskaya Gazeta, the Russian government’s official newspaper.
• It is equipped with cold and heated rooms, electrical substations, treatment facilities, water and fuel storage tanks, and more than 100 fire ponds, the newspaper said.
• The arsenal was put into operation in 2018, then-deputy Defence Minister Dmitry Bulgakov told RIA. Bulgakov was arrested earlier this year on corruption charges, which he denies.
• Bulgakov said in 2018 that the arsenal is equipped with concrete facilities for storing missiles, ammunition and explosives in the proper conditions, RIA reported.
• Bulgakov told Russian army news outlet Zvezda in 2018 that the full load of each storage facility of the arsenal is up to 240 tonnes.
• The arsenal also has facilities to accommodate up to 200 servicemen, the RIA report says.

The US and UK have meanwhile been in discussions about greenlighting use of long-range missiles against Russia. The US administration has been cautious in the face of Putin's red lines.