May 30 - A 33-year-old Michigan woman and convert to Islam has been killed in Syria while fighting with opposition forces against the government of President Bashar al-Assad in the country's civil war, her family said on Thursday.

The woman's aunt told Reuters that the FBI had informed her on Thursday afternoon of the death of her niece, Nicole Mansfield of Flint, but said she did not have the details of how she died.

"I'm just devastated," said the aunt, Monica Mansfield Speelman. "Evidently, she was fighting with opposition forces."

Speelman said Mansfield, a single mother of an 18-year-old daughter, had converted to Islam about five years ago but that she did not know when her niece had traveled to Syria.

"I didn't think she would stoop that low to go over there and try to harm anybody," Speelman said of her niece, who she said had worked at a group home.

A US State Department official, speaking on condition of anonymity, had earlier told Reuters that Washington was working via the Czech Republic mission in Syria to get more information.

"As we do in all such cases, we are working through our Czech protecting power in Syria to obtain more information, and we appreciate the efforts of the Czech mission on behalf of our citizens," the official said. He added that US authorities could not comment further "because of privacy considerations".