massive dust storm has descended on the Phoenix area, drastically reducing visibility and delaying flights as strong winds toppled trees and caused power outages for thousands of residents in the valley.The wall of dust towered over city centre skyscrapers after it swept across the desert from the south, and KSAZ-TV reported it appeared to be roughly 50 miles wide in some spots.

The dust cloud briefly blanketed downtown Phoenix at around nightfall.

The storm was part of the Arizona monsoon season, which typically starts in mid-June and lasts through September 30.

The National Weather Service said strong winds with gusts of more than 60mph rapidly moved the dust cloud north west through Phoenix and the cities of Avondale, Tempe and Scottsdale. More than a dozen communities in the area were placed under a severe thunderstorm watch.

About 8,000 Salt River Project utility customers were left without power, KNXV-TV reported late on Tuesday.

The Arizona Republic reported winds downed live wires in Tempe that sparked a fire at a busy road junction, but firefighters were able to quickly extinguish the blaze.

The Federal Aviation Administration said on its website that because of low visibility in the area, no Phoenix-bound flights were allowed to leave Las Vegas or Los Angeles airports, and flights at the airport were grounded for about an hour.