At least 70 soldiers have been killed after a suicide bomber struck an Iraqi military convoy east of Mosul, according to Kurdish news agency Firat.
The attack took place near the village of Bilawet and the bomber was a member of the Islamic State, the news agency claims.
Iraqi soldiers were advancing towards Mosul to capture it from Isis, the last major stronghold occupied by the Islamist group.
Isis seized Mosul in 2014, and the UN has expressed concern for civilians who may be caught up in the conflict, which they believe could last for months.
In the first few hours of the operation the Kurds had captured several villages and pro-government forces have made gains from the south, according to security sources.