The area was identified by US intelligence officials.
"That zone is where the enemy is thickest," CNN quoted Col. Steve Warren, chief spokesman for the coalition as saying on Wednesday.
"We've seen some of the leaders move their families, we believe, into Tal Afar as we've placed increased pressure on Mosul," Warren said.
"That zone is where the enemy is thickest. And personnel, leaders, equipment, information, money is moving, you know, back and forth within that corridor all the time, continuously. That's where they are."
US warplanes strike targets in the area when they are able to find them, Warren said.