The search for a missing hiker in California's Sequoia and Kings Canyon National Parks is being scaled back after no clues or leads regarding the man's location have emerged.

National Park Service (NPS) officials said in a statement that the search for 66-year-old Quang Trong Than has been scaled back to "limited continuous mode," meaning the search is still active but authorities are reducing the resources that have so far been dedicated to it.

The hunt for Than is centered on the area around Split Mountain—a 14,000-foot-plus peak that lies at the boundary between Kings Canyon and Inyo National Forest.

Sequoia and Kings Canyon parks, which have been jointly administered since 1943, cover an area of more than 1,300 square miles in California's Sierra Nevada. These parks feature a wide variety of ecosystems and contain several peaks over 14,000 feet high, as well as giant sequoia trees.

Than, who is from Newport Beach, California, was last seen on August 21 between 3 and 4 p.m. near the peak of Split Mountain, according to the NPS.

He was with two others, and they were planning to hike to the summit of Split Mountain before exiting through Inyo National Forest. As a result, Than did not have any overnight gear on him.

At the base of the summit, Than became separated from his companions, according to the Inyo County Sheriff's Office.

"At 13,200 feet one party member waited 60-90 minutes for Quang and the other party member. Quang 'waved them on' indicating he would either fall behind, wait for them there, or head down. No plan was made," the sheriff's office said in a statement.

"Quang had stashed a helmet and jacket at Red Lake and had not returned to pick it up, and the other party members did not see him on their descent," the statement said.

The other hikers then reported that Than was missing, and a search and rescue operation was launched on August 22.

The massive search effort has involved officials from Sequoia and Kings Canyon parks, the Inyo County Sheriff's Department, Inyo National Forest, Inyo County Search and Rescue, the California National Guard, Sierra Madre Search and Rescue, the Air Force Rescue Coordination Center, the Tulare County Sheriff's Department and Yosemite National Park.

Teams on the ground and in helicopters have been searching for Than with the help of infrared imaging, drones and dogs. Officials have also questioned hikers in the area where the 66-year-old was last seen.

But according to the NPS, "no clues or leads have given any indication as to Than's whereabouts" at this time.

At left, a photo taken by searchers on a reconnaissance helicopter flight shows California's Split Mountain in the Sierra Nevada. At right, Quang Trong Than is pictured near the mountain's peak on August 21, when he was last seen.Sequoia and Kings Canyon National Parks/Inyo County Sheriff's Office

"Than's itinerary was extremely challenging, and few other hikers were in the area," the NPS said.

Sierra Madre Search and Rescue Team said the route that Than was taking up the mountain is "quite arduous," with an elevation gain of around 7,500 feet over roughly 7 miles and some "hazardous" features.

According to the sheriff's office, search efforts have been made challenging by the high elevation in the area where Than was last seen and the "extremely hot weather."

While efforts to find Than are continuing to some extent, they will be significantly scaled back and teams will no longer be actively searching the terrain, the sheriff's office said Wednesday. Instead, it will focus on "new clues and new points of interest."

"The plan is to continue to review new information based on analysis of drone footage and respond accordingly," the office said.

The NPS is urging anyone who was in the area where Than was last seen on or around August 21 to contact the agency's Investigative Services at 888-653-0009 or nps_isb@nps.gov.

The agency described him as 5 feet, 2 inches tall and weighing 145 pounds, with gray hair and brown eyes.

Newsweek has contacted the NPS for further comment.