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Looks like it could be a small nuke.
Current technology allows them to be quite small.
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For what purpose? Ukraine who supposedly has no nukes, drops it and opens the door to all out Russian invasion of everything including Kiev. No way the west would back them up after a first use for a trival reason.
And the Russians? Why would they drop on a city their side controls?
Just looks like a big battlefield explosion. They do happen. Look the Marines at Roi-Namur. A grunt threw a satchel charge into a concrete building. Turned out to be a very large storage site for torpedo warheads. The photos look like the whole island went up in one blast.