Six people were killed and one was seriously injured on Sunday when two medical helicopters collided on their way to a hospital in Flagstaff, Arizona, air officials said.

The helicopters crashed near Flagstaff at about 3.45 p.m. PDT (11:45 p.m. British time), said Ian Gregor, a spokesman with the Federal Aviation Administration in Los Angeles.

Preliminary reports indicated at least one patient was among the three people killed in a helicopter operated by Air Methods Corp, Gregor said. The other four fatalities, as well as the injury, occurred aboard a helicopter operated by Classic Helicopter of Utah, he added.

A spokesman for the Flagstaff Police Dept. said two rescue workers were slightly injured in a secondary blast as one of the helicopters on the ground exploded.

"They were treated for minor burns injuries and were released from the hospital in good condition," Sergeant Tom Boughner told Reuters by telephone from the crash site.

Boughner said the two helicopters slammed into a hillside covered with ponderosa pines, leaving a large field of scattered debris and sparking a fire.

The blaze torched about 10 to 15 acres (4 to 6 hectares), but was quickly contained.

Flagstaff lies about 75 miles (120 km) south of the Grand Canyon National Park, one of the largest tourist draws in the United States, which attracts some 5 million visitors a year.

Four people were killed in a similar accident in Arizona last year, when two news television helicopters collided while following a car chase in Phoenix.