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What caused this enormous explosion in the Syrian city of Homs?

A video uploaded by Syrian activists purports to show an explosion over Homs. (YouTube)

An unverified video uploaded Thursday by Syrian activists appears to show an explosion many stories high in the war-torn Syrian city of Homs. It's difficult to tell for sure how high the explosion is, but it dwarfs an apartment complex in the foreground that is at least seven stories.

The footage, embedded just below, captures a fireball fulminating over several long seconds before darkening into a large, mushroom-shaped cloud. A secondary explosion, small only compared to the initial blast, is also visible.

The explosion was also reported by the London-based anti-Assad group Syrian Observatory for Human Rights, which told the BBC that the blast was the result of rebel rocket strikes on a weapons depot. The incident, they said, caused 40 deaths.

The explosion has been discussed heavily by social media users within Syria. Speculation, it's not clear how informed, made before the Syrian Observatory announcement had turned on the theory that the blast was the result of a rocket attack or other sort of explosion against an arms depot. It's not clear whether the Syrian Observatory report is independent of or based on social media discussion.

The past month has seen extremely heavy fighting in Homs, which in 2011 was the site of some of the Assad regime's most brutal crackdowns on civilian protesters. Syrian military forces have been fighting since early July to retake Homs and had recaptured a key district Monday, the BBC reports.