(Adds details from witness account, background)
BAGHDAD, Aug 22 (Reuters) - Islamic State militants in Iraq stoned a man to death in the northern city of Mosul after one of their courts sentenced him to die for the crime of adultery, a witness said on Friday.
The stoning, which happened on Thursday, was the first known instance of the punishment by al Qaeda offshoot Islamic State militants in Iraq, although it had been reported previously in Syria.
Members of the hardline Sunni group, which seized Mosul in June and also controls swathes of territory in neighbouring Syria, have shot, beheaded and crucified prisoners and blown up and bulldozed Shi'ite mosques.
The United States began air strikes on Islamic State positions in Iraq this month, and in retaliation the group published a video of one of its black-clad fighters beheading U.S. journalist James Foley.
A taxi driver who said he saw Thursday's stoning in Mosul said he stopped to find out what was happening when he noticed a crowd gathering and about 20 Islamic State vehicles parked nearby.
Some militants were gathering sharp pieces of rock while another held a video camera, he said.
Eventually, the militants brought a handcuffed young man from one of the vehicles, made him sit down on the ground, and began pelting him with rocks, the witness said. Some in the crowd turned away as the man cried, "I did nothing," he said.
"I had a glance at this person, I saw some of the stones hitting his head, and then I turned away," the witness said, asking not to be named out of fear of retaliation.
After about 15 minutes, the militants told the crowd the stoning was finished, and an ambulance took away the body. A source at the morgue in Mosul confirmed the death of the man and said his identification showed he was 30 years old.
The stoning was also reported on local media. Neither the witness's nor the media's accounts could be independently corroborated.
In July, activists reported Islamic State militants stoned two women to death for adultery in Syria's Raqqa province, which borders Turkey and is a major stronghold for the group.
U.S.-trained Iraqi government forces, which largely fled when Islamic State militants arrived from Syria, have not been able to push their way back into Mosul, where the group has imposed its extreme ideology, even destroying statues of famous Iraqi poets it deems as un-Islamic.
The witness said the stoning was widely discussed on the streets of Mosul, which was seen as one Iraq's most tolerant cities before falling under the grip of the group, which is made up of Iraqis and other Arabs as well as foreign fighters.
Many sects and ethnic groups lived in harmony in the region. Now all sides are just trying to survive the reign off the Islamic State.
"Most of the people were not surprised, because this is typical of their behaviour," he said, referring to the militants.
(Reporting by Raheem Salman; Writing by Alexander Dziadosz; Editing by Michael Georgy)