Death toll from the flash floods early Friday in the U.S. state of Arkansas continued to rise, with authorities putting the latest fatalities at 20.

State police initially said 12 people were killed after floods washed away a camp site in western Arkansas. Later on, the figure climbed to 16.

CNN quoted state authorities as reporting that the latest death toll has reached 20.

Another 36 went missing after the floods, triggered by heavy rains, swamped the rugged Albert Pike campground area of western Arkansas, according to CNN.

Arkansas state police spokesman Bill Sadler quoted by CNN, said that a search is underway for people still trapped in the area, a relatively remote and rural region where cell phone service could be spotty.

Arkansas Governor Mike Beebe said he had learnt from the Red Cross that there could have been as many as 300 people in the flooded area.

"The primary mission of the Arkansas State Police, working with local authorities right now, is to get the living out of the area and locate the dead," Sadler said.

Two helicopters are at the scene helping with the search-and-rescue efforts, Sadler said. A temporary morgue has been set up.

Beebe said he suspects the people were killed around 5:30 or 6 a.m. local time Friday, possibly with no idea the fast-rising waters were coming. "They probably had no warning," he said.

He also said he didn't know how many of the victims were adults and how many were children. "That's one of the things that concerns me."

The National Weather Service says 7.6 inches of rain slammed the campgrounds. The Little Missouri River rose from about 3 feet Thursday night to more than 20 feet early Friday, according to Beebe.

"It was a very rapid flash flood that inundated that area," he said, "It's an unmanned campground in terms of being a campground with all the amenities."

U.S. senator Blanche Lincoln, from Arkansas, issued a statement expressing condolences to families of the victims.

"My heartfelt condolences to everyone who has been affected by this tragic weather event. I will continue to be in contact with state and local officials to monitor the progress of the situation, " she said.