Image copyright Joshua Tree Search & Rescue Image caption Rescuers have faced danger, as temperatures soar into triple digits fahrenheit

A search is under way for two people who have been missing in California's Joshua Tree National Park since Friday.

Search crews aided by helicopters and teams of dogs have struggled to continue the search, which comes amid a wave of temperatures above 100F (37C).

Some rescuers have needed to be airlifted to safety after succumbing to the excessive heat and humidity.

Rachel Nguyen, 20, and Joseph Orbeso, 21, vanished while hiking to celebrate Ms Nguyen's birthday.

After they failed to check out of their Airbnb accomodation on Friday, leaving all their belongings behind, a search began.

Image copyright Joshua Tree Search & Rescue Image caption Some rescue crews have required evacuation due to the extreme heat

Hundreds of volunteers with Joshua Tree Search & Rescue, as well as six teams of search dogs, have been combing among the distinctive trees and desert formations for the two students.

Their vehicle was found on Friday near the Maze Loop trail after a ping from Mr Orbeso's phone was detected by investigators.

Volunteers with Joshua Tree Search & Rescue thought they had detected footprints on a trail leading north into a canyon, but they seem to have lost the tracks.

Image copyright National Park Service Image caption The pair have not been seen since last Thursday

"There were some footprints. We found them in sporadic places through intermittent areas through the park," said George Land, Joshua Tree National Park spokesman.

"One thing that will happen when people are in heavy heat stress, they'll start taking off pieces of clothing. We haven't found any of that thus far," he added.

Mr Land added that two fixed-wing planes and two helicopters equipped with thermal imaging have been flying over the park, but so far no sign of them has been found.