isis fighters destroyed ancient ruins of iraqi assyrian city of nimrud dating back to the 13th century b.c.
the attack , near mosul , took place last month , but a seven-minute video of destruction has now emerged .
it shows militants hacking and drilling away at 3,000-year-old relics and blowing up the ancient ruins .
the u.n. secretary-general ban ki-moon has called the destruction of the nimrud ruins ` a war crime '