The death toll from a series of tornadoes that swept through the US state of Oklahoma late last week has risen to 18 and includes three storm chasers, officials said Monday.

Six children are also among the fatalities from late Friday's twisters, said the chief medical examiner's office, which had previously placed the number of victims at 14.

Storm chasers Tim Samaras, his son Paul and their partner Carl Young were caught in a twister that struck El Reno, west of Oklahoma City, one of several that hit a region still reeling from a huge tornado that claimed two dozen lives last month.

They were the first storm researchers ever to lose their lives while pursuing twisters, media reports said, citing the National Weather Service's Storm Prediction Center in Norman, Oklahoma.

"They all unfortunately passed away but doing what they LOVED," the elder Samaras's brother Jim Samaras said on his Facebook page.

Samaras's instruments are said to have offered the first-ever glimpse inside a tornado, and his Tactical Weather Instrumented Sampling in Tornadoes Experiment, or TWISTEX, aimed to learn more about the storms in order to help increase the lead time for warnings.

"We still don't know why some thunderstorms create tornadoes while others don't," Samaras told National Geographic in one of his last interviews.

"We're trying to collect as many observations as possible, both from outside and from the inside."

Debris from the chasers' vehicle was strewn across about half a mile (0.8 kilometers), Canadian County Undersheriff Chris West told AFP.

One of the bodies was recovered from the vehicle, while the two others were found about a quarter of a mile in either direction.

Crews hauled away a badly mangled white truck with its windows smashed and its body crushed and twisted almost beyond recognition.

"Tim was a courageous and brilliant scientist who fearlessly pursued tornadoes and lightning in the field in an effort to better understand these phenomena," National Geographic executive vice president Terry Garcia said.

It said he developed his interest in twisters after watching the "The Wizard of Oz" when he was just six years old. The movie begins with a tornado sweeping heroine Dorothy and her dog Toto away to the magical Land of Oz.

"Though we sometimes take it for granted, Tim's death is a stark reminder of the risks encountered regularly by the men and women who work for us," Garcia said in a statement.

Samaras developed probes to measure the environment inside tornadoes. Researchers had to place the devices in the path of the storm and then escape before being swept away.

He measured the lowest barometric pressure drop ever recorded (100 millibars) at a tornado's center, saying that was equivalent to "stepping into an elevator and hurtling up 1,000 feet (305 meters) in 10 seconds."

Samaras was also a star on the Discovery Channel's show "Storm Chasers," which ended last year.

His brother Jim Samaras said the storm chaser "looked at tornadoes not for the spotlight of TV, but for the scientific aspect."

"At the end of the day, he wanted to save lives and he gave the ultimate sacrifice for that," he told The Denver Post.

The storm-chaser's final message on Twitter warned: "Storms now initiating south of Watonga along triple point. Dangerous day ahead for OK (Oklahoma) -- stay weather savvy!"

Despite the big risks, Samaras was known to be very cautious.

"He knew where not to be and in this case the tornado took a clear turn toward them," Jim Samaras said.