Rescue operations are under way after part of an apartment building collapsed in the eastern Iowa city of Davenport.

Authorities have not said whether anyone was killed, adding that an unspecified number of people were treated for injuries at the scene.

Rescuers were called to the scene shortly before 5pm on Sunday local time.

Fire crews rescued seven people and escorted more than a dozen others from the building in their initial response, Davenport fire chief Michael Carlsten said at a news conference.

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Mr Carlsten said the back of the six-storey apartment complex collapsed and had separated from the building, which houses apartments on the upper floors and businesses on the ground level.

Authorities found a gas leak after the collapse, Mr Carlsten said, while water also had leaked throughout the floors of the structure.

First responders were still searching for people who were unaccounted for on Sunday. The stability of the building was a concern following secondary collapses while rescuers were at the scene, Mr Carlsten said.

“Our focus is rescue right now,” Davenport mayor Mike Matson said at a news conference.

“This is an active scene. We will continue to work, continue to evaluate, with the whole purpose of trying to find people and trying to get them out,” Mr Matson said.

The cause of the collapse is not immediately known.