German authorities say they are providing assistance to the 16-year-old.
Human Rights Watch said an Iraqi unit trained by American forces summarily executed prisoners in western Mosul.
German security services say more than 900 German nationals have left the country to fight for ISIS.
Canadian nationals were reportedly among those who joined the militant group in Iraq.
Iraqi security forces discovered 16-year-old Linda W. from Saxony among five German ISIS members.
The report said civilians were subjected to "relentless and unlawful attacks" by the coalition during the Mosul military operation.
Iraqi forces are making women remove their headscarves after 38 female suicide bombers struck over the past month.
Former Iraqi envoy to the U.S. asks: Do Iraqis want a united or divided country?
U.S. Army Major General Joseph Martin, head of ground forces for the U.S.-led coalition fighting ISIS, tells Newsweek of the complex offensive that Iraqi forces are facing.
The battle for Mosul is approaching its end after seven months of combat.
The pontiff expressed sadness over deaths and said the U.S.-led coalition had an obligation to protect civilians.
Stephen Townsend said increases in civilians casualties are to be expected.
An Amnesty report cites two airstrikes in Mosul, which allegedly killed up to 300 people.
At least four prisoners have died, Human Rights Watch claims, in cases apparently linked to lack of proper medical care.
Forces recaptured the city's government complex as part of a new drive into western Mosul.
The key road links the northern Iraqi city to Syria.
The shrine was a reported burial site of a prophet known in the Koran as Yunus and in the Bible as Jonah.
A coalition of Iraqi forces is continuing its fight to oust the group from the northern city.
Searches are underway to establish the whereabouts of the troops.
Civilians escaping from ISIS-held territory in northern Iraq find that justice is not always forthcoming.
Iranian-backed Shi'ite paramilitaries have linked with Kurdish forces west of the city.