US warplanes today attacked and destroyed the Number Four Bridge in the northern Iraqi city of Mosul, a move which Pentagon officials say was meant to limit the ability of ISIS forces, who presently control the city, to move back and forth across the Tigris River.

This is the southernmost of five bridges in Mosul, and the third destroyed by the US this month. UN officials are expressing concern that the strikes, in addition to limiting ISIS militarily, might also leave hundreds of thousands of civilians with no easy way to flee the fighting.

Iraqi officials have said that their plan in the Mosul offensive in to take the eastern shore of the river first before advancing into the west. The limited presence Iraq has inside the city is on the edge of the eastern part of the city.

Iraqi forces have been public about firing on any vehicle that moves in the eastern part of the city, and militias outside the city in the west have promised to target “fleeing rebels,” meaning it isn’t clear what routes will be reliable for civilians to flee from. The lack of bridges means that it’s going to be harder and harder to get out of those limited areas.