ISIS fighters forced women, children and the elderly to walk alongside them for days as human shields to cover their retreat to Mosul, separating out older boys and fighting-aged men along the way for an unknown fate, villagers said.

Reuters spoke to a woman and an elderly man inside the ISIS-held city, who were part of group of families forced to leave the villages of Safiya and Ellezaga, about 30 miles to the south.

Children and the elderly were released when they arrived in Mosul on Tuesday and told to stay with relatives, they said, speaking by phone from one of the few places where there is still mobile coverage, on the city’s edges.

A resident of Mosul, Rayyan, said he saw the families when they arrived in the city, “their bare feet bleeding and covered with dust as if coming from under the rubble.”

“We cried when we saw them,” he said.

The Iraqi ground offensive to retake Mosul is expected to be the biggest battle in Iraq since the U.S.-led invasion in 2003.

For the men, the walk ended in Hammam al-Alil, a town about 15 10 miles south of Mosul where Iraqi officials say ISIS is killing former members of the police and army who had lived in areas under its control.

They haven’t been heard of since, their relatives said.

Abu Ahmed, in his early sixties, said the militants didn’t use cars in their retreat, fearing air strikes.

He walked with his daughters and his grandchildren from Ellazaga, sleeping two days in the open. The militants took away one of his sons.