WASHINGTON—U.S. forces have entered Mosul to assist Iraqi troops in retaking the besieged city, the Pentagon said Wednesday, part of an increasing American presence in the fight against Islamic State militants.

Advisers from U.S. and coalition partner forces have stepped up their activity in Iraq, approximately doubling to 450 the number working directly with local forces in the Mosul offensive. Those troops have gone into the city with Iraqi forces, the Pentagon said.

...