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A Florida man and his teenage stepson died after hiking during extreme heat at Big Bend National Park in Texas, officials said.

The boy fell ill along the trail and lost consciousness, and his stepfather was killed in a car accident as he tried to find help, authorities said.

The exact causes of their deaths was not immediately known.

Texas and other southern states have been experiencing a historic heat wave in recent weeks.

The National Park Service has cautioned hikers to stay off trails in the afternoon, stay hydrated and limit sun exposure.

The man, 31, and his two stepsons were hiking along the Marufo Vega Trail on Friday in 119F (48C) heat when the 14-year-old fell ill.

While the stepfather hiked back to his vehicle to seek help, the teen's 21-year-old brother tried to carry him back to the trailhead, according to the National Park Service.

The Big Bend National Park's Communications Center received a call requesting emergency assistance at about 18:00 local time (23:00 GMT).

"The Marufo Vega Trail winds through extremely rugged desert and rocky cliffs within the hottest part of Big Bend National Park," the park service said. "No shade or water makes this strenuous trail dangerous to attempt in the heat of summer."

Park rangers, and US Border Patrol Agents, arrived at around 19:30 local time and discovered the teen along the trail, dead.

Rescuers searched for the father and, at about 20:00 local time found his vehicle had crashed over the embankment at the Boquillas Overlook. He was pronounced dead at the scene of the crash.

The names of the two victims were not immediately released by authorities.

The 21-year-old brother was unharmed, but there were no other details on his condition.

Big Bend is currently experiencing extreme heat with daily highs reaching 110F-119F at low elevations and along the Rio Grande, according to officials.