Flight 812 declared an emergency "due to loss of pressurization in the cabin" and landed safely in Yuma, Arizona about 40 minutes after takeoff, Southwest said in a statement.

"Upon safely landing in Yuma, the flight crew discovered a hole in the top of the aircraft," the Southwest statement said.

None of the 118 passengers were hurt but one flight attendant "received a minor injury upon descent," the airline's statement said.

The flight departed from Phoenix, Arizona at about 3:25 pm (1025 GMT) and was bound for Sacramento, California.

The plane landed at Yuma Marine Corps Air Station/International Airport at 4:07 pm (1107 GMT), Federal Aviation Administration spokesman Ian Gregor said. Yuma is 185 miles west of Phoenix and located near the Mexican border.

"We do not know the cause of the decompression," Gregor said. "People on the ground in Yuma are reporting there's a hole in the fuselage."

An FAA inspector from Phoenix was en route to Yuma, he said. The National Transportation Safety Board also was investigating.