More than 20 people may be missing or unaccounted for following a flash flood in Grand Canyon National Park in the US state of Arizona, authorities said Sunday.

The "significant flash-flood event" occurred Saturday in Bright Angel Canyon and the Phantom Ranch area, the US National Park Service said in a news release. It urged anyone who knew of hikers or backpackers who were in the inner canyon along the Bright Angel Creek corridor on August 29, as well as anyone with a campground reservation in the affected area, to provide information to the National Park Service Investigative Services Branch.

Park officials said they were continuing evacuation and response efforts in coordination with the Arizona Department of Public Safety while working to account for people who may have been in the affected areas.

In an earlier news release, park officials said 62 people had been evacuated from the area as of Sunday morning, with additional evacuations scheduled for later in the day.

The flash flood caused significant infrastructure damage throughout Bright Angel Canyon. Nearly all footbridges over Bright Angel Creek were destroyed, preventing hikers from crossing the creek, according to park officials.

Parts of Grand Canyon National Park have been closed, and the Colorado River has been closed to river traffic until further notice because metal structures and debris entered the river, officials said.

Officials also warned that additional thunderstorms and heavy rainfall were possible Sunday and Monday, potentially causing further flash flooding, debris flows and rockfalls, as well as changing trail and river conditions.