PHOENIX • At least nine people drowned while swimming in an Arizona river in the United States when they became overwhelmed by flash flooding following heavy rains in the drought-stricken region, officials said on Sunday.

The authorities were searching for at least one other missing person after the incident last Saturday afternoon in the Verde River near Payson, about 145km north-east of Phoenix, Water Wheel Fire and Medical District fire chief Ron Sattelmaier said.

Local media reported that at least two children were among the dead.

The Gila County Sheriff's Office did not give details of the dead or those missing.

A search-and-rescue operation was under way, involving sheriff's deputies and the Arizona Department of Public Safety Ranger Helicopter, as well as members of the local Whispering Pines Fire Department and US Forest Service, Sheriff Adam Shepherd said in a statement.

The victims were enjoying the relatively cool temperatures north of Payson on a day trip, said Sheriff's Office spokesman David Hornung, adding that the area is a popular getaway for people from southern Arizona. He referred to the gathering as a "family group", though he said some members of the party were not related.

The storm hit on Saturday, the first day of the region's monsoon season, which lasts from mid-July till September. Flash flooding is common in Arizona during those months.

The state's low-lying roads and dry rivers and creek beds are particularly vulnerable to sudden downpours, with vegetation too sparse to slow the water's sudden onset.

Along with the flooding, last Saturday's storm caused several power failures, leaving thousands of people without electricity.

The National Weather Service in Phoenix said that most of Arizona was under flash-flood watch until yesterday evening.

REUTERS, NYTIMES