The case against Jennifer Wenisch, 27, is believed to be the first anywhere in the world for international crimes committed by IS militants against members of the Yazidi minority.

She faces a maximum term of life in jail if found guilty of committing murder and of murder as a war crime, as well membership in a terrorist organisation and violations of the German War Weapons Control Act.

It is Germany's first trial of a female IS returnee, prosecutor Claudia Gorf told the Munich court.

Wenisch -- wearing a white blouse and black jacket, her dark hair not covered -- showed no emotion and did not speak, but shielded her face with a paper folder while photographers were in the room at the start.

Nobel Peace Prize Laureate Nadia Murad, herself a Yazidi survivor of IS enslavement and torture, said in a statement that the trial "is a very big moment for me and for the entire Yazidi community".

Prominent London-based human rights lawyer Amal Clooney is part of the team representing the dead Yazidi girl's mother, although Clooney did not appear on the trial's opening day.

- 'Agonising death' -

German prosecutors allege Wenisch and her IS husband "purchased" the Yazidi woman and child as household "slaves" whom they held captive while living in then IS-occupied Mosul, Iraq, in 2015.

"After the girl fell ill and wet her mattress, the husband of the accused chained her up outside as punishment and let the child die an agonising death of thirst in the scorching heat," prosecutors charge.

"The accused allowed her husband to do so and did nothing to save the girl."

German media said the defendant's husband, Taha Sabah Noori Al-J., had beaten both the mother and child, and that Wenisch allegedly also once held a pistol to the woman's head.

The trial is being held under tight security in a court for state security and terrorism cases, with hearings initially scheduled until September 30.

- Morality police -

Wenisch -- who reportedly left school after the eighth grade and has no job or qualifications -- converted to Islam in 2013 and travelled the following year via Turkey and Syria to Iraq where she joined the IS.

Recruited in mid-2015 to the group's self-styled hisbah morality police, she patrolled city parks in IS-occupied Fallujah and Mosul.

Armed with an AK-47 assault rifle, a pistol and an explosives vest, her task was to ensure strict IS rules on dress code, public behaviour and bans on alcohol and tobacco.

In January 2016, months after the Yazidi child's death, she visited the German embassy in Ankara to apply for new identity papers. When she left the mission, she was arrested and extradited days later to Germany.

For lack of actionable evidence against her at the time, she was allowed to return to her home in the German state of Lower Saxony, but quickly sought to return to IS territory.

- FBI informant -

Der Spiegel reported that an FBI informant posed as an accomplice who offered to take Wenisch and her two-year-old child back to the IS "caliphate".

While they were sitting in a bugged car, headed for Turkey, Wenisch allegedly spoke of her time at the IS and incriminated herself.

She said that the child's death had been "hard-core even for the IS" and unjust because only God had the right to use fire as punishment, adding that her husband had later been beaten as punishment by the IS.

Police followed her car for several hours and listened to a live audio feed, then arrested Wenisch at a highway stop.

Amal Clooney, the wife of Hollywood star George Clooney, has been involved in a campaign to get IS crimes against the Yazidi recognised as a "genocide".

"I hope this will be the first of many trials that will finally bring ISIS to justice in line with international law," the lawyer said in a statement, using an alternative acronym for the group.