LOS ANGELES - Four passengers and a pilot were killed when a helicopter crashed near Las Vegas Wednesday evening, a spokesman for the National Park Service said.

The accident occurred around 5 pm local time (GMT 0100) when the aircraft, operated by Sundance Helicopters, was taking passengers on a tour over Hoover Dam and went down outside of the Lake Mead National Recreation Area, east of Las Vegas, authorities quoted by local media said.

Several witnesses reported smoke and what appeared to be wreckage in the area, KTNV reported.

Police spokesman Bill Cassell said rescuers were trying to reach the isolated area of the crash, but later the rescue mission turned to a recovery effort after it was discovered that the five on board were killed. Rescuers said they may have to wait until the morning to continue the recovery effort due to the location of the crash.

The cause of the accident was still unknown.

The helicopter was an Aerospatiale AS350, which can hold up to six passengers and are often used for air tours.

Founded in 1985, Sundance Helicopters operates the largest and most experienced helicopter company in Las Vegas, featuring daily Grand Canyon Helicopter Tours. The company says on its website that it carries "more than 160,000 customers safely each year".

In September 2003, a Sundance Grand Canyon tour helicopter crashed, killing all seven people on board, including at least four international tourists.