DADEVILLE, Ala.—An outdoor deck collapsed at a house in Alabama, seriously injuring at least six people, authorities said.

It wasn’t clear how high the deck was or how far people fell when it collapsed Sunday at the home near Lake Martin, a large reservoir northeast of Montgomery that is a popular recreation area.

It wasn’t immediately clear why the deck collapsed, said Ben Wilkerson, chief of a volunteer fire and rescue department in the area. About 15 people were on it at the time. Everyone is expected to recover.

When emergency crews first responded, “it was a little chaotic,” Mr. Wilkerson said. Other fire departments and ambulance services were called.