ERBIL, Iraq (Reuters) - Two mass graves of at least 18 members of Iraq’s Yazidi minority, thousands of whom have been killed and kidnapped by Islamic State, have been discovered as security forces fight to dislodge the militants from Mosul, a local official said.

Kurdish peshmerga forces found the grave near the Shababit junction in northwestern Iraq while scouting the area. It contained bones and identity cards that appeared to have been covered over with sandy earth by a bulldozer.

Islamic State systematically killed, captured and enslaved thousands of Yazidis in the summer of 2014 as they overran the Sinjar area, where many of them lived. United Nations investigators have said that constitutes genocide.

The mayor of Sinjar, Mahma Xelil, said the latest discovery brought the number of Yazidi mass graves found so far to 29, estimating the total would rise to more than 40 as the militants are driven back further.

Numbering about 400,000 people, Yazidis are a religious sect whose beliefs combine elements of several ancient Middle Eastern religions and are considered devil-worshippers by the hardline Sunni Islamist insurgents.

The Office of Kidnapped Affairs in Duhok, a department backed by the Kurdistan regional government, says about 3,500 Yazidis are believed to still live in areas controlled by Islamic State, many of them women and children.

Last Wednesday, 18 escaped from the town of Tal Afar in northern Iraq as Shi’ite paramilitaries cut it off from the south and west.