KATHMANDU, Nepal (AP) — Rescue teams with search dogs combed through mud, rocks and debris Tuesday in Nepal and China looking for thousands of people still missing after catastrophic flooding tore through communities across the Himalayas, leaving more than 1,000 dead.

Nepal’s disaster agency said Tuesday that 987 deaths had been counted there, with 3,916 people missing, including 583 foreigners. Chinese authorities, in their latest update on Sunday, reported 16 deaths and 546 people still missing, including 261 foreigners.

The scale of last week's disaster was hampering rescue efforts in both regions, with landslides, destroyed roads and widespread damage cutting off remote areas. Crews faced challenging conditions as they searched unstable terrain and piles of debris, raising fears the death toll could rise as rescuers reach isolated communities.

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Rescuers race to reach missing hydropower workers

In Nepal, rescuers were racing to reach hundreds of workers believed to be trapped inside tunnels at hydropower projects damaged by the floods. About 900 workers are missing from 12 projects, with roughly 500 believed to be trapped in tunnels, according to Nepalese authorities.

(MORE: Raging floodwaters destroy Nepal bridge)

Brig. Gen. Raja Ram Basnet, Nepal’s army spokesperson, said teams assisted by foreign experts were working at several hydropower projects. Visuals released by the army showed rescue teams entering dark, debris-filled tunnels, wading through mud and water and climbing over rocks as they searched for signs of life.

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On the Chinese side, rescue crews scoured areas hit by mudslides, particularly around the Gyirong Port border crossing, which was destroyed.

Some rescuers, aided by search dogs, searched gaps among rocks and debris for signs of life. Other teams used heavy machinery to clear a major highway leading to Gyirong Port, hoping to clear the way for equipment, personnel and supplies to reach the disaster zone, according to China's state-run Global Times newspaper.

Relief operations reach stranded communities

At least 11,814 people have been rescued so far, according to Nepalese authorities. Emergency crews were using helicopters and other aircraft to reach stranded communities in areas where roads and bridges have been destroyed by the flooding and landslides.

Authorities on Tuesday also stepped up efforts to provide assistance to survivors in areas where roads and other transportation routes had been damaged or cut off.

Chinese Foreign Ministry spokesperson Guo Jiakun said China’s second batch of relief supplies for Nepal was expected to ship Tuesday, including drones, DNA testing and water purification equipment. Chinese DNA testing experts will also help Nepal identify bodies, he said.

Bimal Sharma, a helicopter pilot who has been flying rescue and relief missions since the disaster began, said he was shocked to see the devastation in Timure, a border town close to Tibet that he has known for years.

“It felt like I was flying over an entirely different landscape,” he said.

Sharma said he has had little opportunity to speak with the people he rescued, but their expressions told him how deeply the disaster had affected them.

“I can see the pain in their eyes. It’s heartbreaking,” he said.

Disaster began with a glacial collapse

The Aug. 26 disaster began with a chain of events high in the Himalayas. A glacial collapse sent large amounts of rock, ice and meltwater into valleys below, triggering powerful floods downstream.

Scientists studying satellite imagery said the collapse appears to have involved the failure of bedrock beneath a glacier in the Himalayan region. The rockfall was powerful enough to register as a magnitude 5.2 seismic event.

(MORE: How did this flood disaster happen?)

The resulting surge of water and debris entered rivers that flow through valleys in Tibet and Nepal, causing them to rise rapidly. Floodwaters swept away homes, buildings, roads and bridges and carried mud and rocks downstream.

Nepal's Prime Minister Balendra Shah said that the disaster highlighted the need for action to combat climate change.

“This incident has indicated that the risks we face in the Himalayan region are increasing with climate change,” Shah said in a social media post Monday night. “Therefore, this region must be alert, and at the same time, it is time for us to strongly raise the issue of disasters caused by climate change before the international community.”

Saaliq reported from New Delhi. Associated Press journalists E. Eduardo Castillo in Beijing, Kanis Leung in Hong Kong and Aijaz Hussain in New Delhi contributed to this report.