(NBC News) ISIS has seen its income drop by 80 percent in two years as it loses territory and the oil and tax revenue that comes with it, according to a study of its finances.

The self-declared caliphate has seen average monthly income plunge from $81 million in the second quarter of 2015 to just $16 million in the same period this year, according to IHS Markit, a global data-monitoring company.

The report came as Iraq declared victory over the terror group in Iraq after government forces recaptured a key mosque in the former ISIS stronghold of Mosul on Thursday.

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