After they seized Mosul two years ago and destroyed the priceless Mesopotamian artifacts in its museum, Islamic State militants found a practical use for the building - they turned it into a tax office.

The outside world learned of the museum's initial fate from a video Islamic State released months later showing its fighters smashing Assyrian, Akkadian, Babylonian, Persian and Roman artifacts, many of them two millennia old or older.

They wanted to destroy any history that did not agree with their extreme version of Sunni Islam.

Iraqi troops took the museum back last month from the militants, who left its once-famous collection in a sorry state.

Remains of an Assyrian winged bull statue, some carved stone coffins, mosaics and two black blocks with Islamic calligraphy are just about all that's left. Smaller pieces from other items litter the floor.

Government forces are still battling the militants just a few hundred meters (yards) away in the Old City, their last stronghold in Iraq, so the rubble-strewn museum is still out of reach for archaeologists to assess the damage.

Apart from soldiers stationed to guard it, a stray cat nibbling at discarded army rations seems to be the building's only inhabitant. Machine gun fire and mortar rounds rang out from a distance as journalists made their way through the museum.

In a basement room under the main exhibition halls, there was a pile of envelopes used to issue orders to pay Islamic tax, one of main sources of funding for the militants.

"The Islamic State ... seeks to levy your duties which were forced by God on the rich people's money," read a message on the envelope stamped with the group's black-and-white flag.

The "Diwan Zakat", or Islamic tax department, then left a space for names and file numbers to identify the payments.

Next to the tax receipts were green leaflets with Koran quotes, from the same department based in "Nineveh Province", whose capital is Mosul.