KATHMANDU, Nepal (AP) — Rescuers pulled out survivors caked in dark brown mud and helicopters lifted stranded people to safety Friday, as Nepal and China raced to respond to catastrophic flash floods that killed nearly 600 people and left almost 2,500 missing.

Rescue teams were put on high alert for more flooding from a new lake that formed high up on the Himalayan mountains after the initial floods on Wednesday. Experts warned that the lake, created after a glacier collapse that triggered the floods, was starting to overflow.

Nepal's army was deployed to help rescue more than 100 people believed trapped inside a tunnel at a hydropower plant located in the country's worst-hit district. Like elsewhere in the disaster zone, their work was complicated by thick layers of mud that covered the site.

The floods have killed 579 in Nepal, the country's disaster-management authority said Friday. Nepali police later put the number of deaths at 553. There was little information on survivors on the Chinese side, where the death toll rose to five Friday as state media showed evacuations of villages and leaders’ efforts to direct the relief response.

More than 3,700 people have been rescued in Nepal so far. Meanwhile, the number of people who were unaccounted for in the country nearly doubled to 1,924 as more local residents were added to the tally. China's state media previously said 558 people were missing on the Chinese side. Hundreds of the missing are foreigners in the area to work, trek or make a pilgrimage to a sacred peak.

Authorities warn of flooding from a new lake

Nepali police issued a fresh alert Friday after receiving information that a dam on the Tibet side had burst. Security personnel and rescue and relief workers were told to stay on high alert and immediately move to a safe location if necessary.

It was unclear if the dam bursting is related to flooding concerns over the lake that formed after the initial flash floods. Chinese broadcaster CCTV said the newly formed lake, located more than 10 kilometers (6 miles) from Gyirong Port, the hard-hit border crossing with Nepal, sits at an elevation of 2,950 meters (9,680 feet.)

Nepali and Chinese authorities earlier warned of more flooding from the lake, which was already overflowing. Rescue workers heading to the disaster zone were ordered to stop temporarily Friday due to concern the lake would release more torrents of water and debris, and people in potentially affected areas were urged to move to safer ground.

Some 680 rescuers eventually reached the hardest-hit area at Gyirong by foot and via rafts. Some were dropped down from drones flown to the area, CCTV footage showed.

Later, monitoring data from the Chinese Ministry of Water Resources showed that the lake had dwindled in size. The water level dropped by 10 meters (32 feet) from the high point on Thursday morning, the state broadcaster said.

Nepali police spokesperson Abi Narayan Kafle told The Associated Press that the rescue operation was continuing.

Thousands of people have been evacuated while others await rescue

In Nepal's Rasuwa district, Nepali army rescuers were struggling to help more than 100 people believed to be trapped by thick mud in a tunnel that's part of the Upper Trishuli-1 Hydropower Project. The army said it had rescued 350 people from the tunnel so far.

Video from the scene showed rescuers helping a man covered in dark brown mud out of the tunnel by pulling and pushing.

Army spokesperson Brig. Gen. Raja Ram Basnet told the AP the focus was on survivors.

“It is complicated to find the entry and exit points of the tunnel because everything has been covered by mud,” Basnet said. “Our first priority is rescuing the stranded people.”

Nepal’s disaster-management authority said 3,253 people had been airlifted by helicopter from the disaster area. Survivors, including injured people, were taken to Kathmandu and other locations.

Some felt the ground shake, then saw a ‘black storm’

Scientists who studied satellite images said bedrock underneath a glacier high in the Himalayan region had collapsed, taking part of the glacier with it. The rockfall was so extreme that it registered as magnitude 5.2. The rocks and melted water then swelled the rivers in the valleys below, causing a torrent of water that swept buildings, bridges and earth downstream in Tibet and Nepal.

Customs officer Karbir Gaire was among some of the survivors in Timure, the closest Nepalese village to the border with China, where some 1,500 people had lived.

Gaire reached his office around 8 a.m. Wednesday and was working on his computer when he first felt the ground shake.

“Right after the shake, I noticed a black storm coming towards us,” he recalled from Kathmandu, where he is back with family. “We all immediately started running out, and within a few seconds, we were climbing uphill on the opposite side of our office building. I think those 10 or so seconds saved some of us.”

Gaire doesn’t remember exactly how many people were in the office when the disaster struck, but 15 colleagues are missing. “It was like a Hollywood movie scene. I don’t call it a flood. It was a tsunami.”

Families around the world hope for news of loved ones

Nepal’s tourism board has said that among the missing are more than 500 foreigners.

While the Nepal government shared a public list of people they rescued, there was little information about victims on the Chinese side of the border.

The flash floods destroyed the road leading to the border, making access to the hardest-hit area difficult. Chinese government officials also stuck to their tight media controls. Information about major disasters is typically released by officials with a delay and without personal stories of victims. China’s Guangxi province, which was devastated by a typhoon in early July, said a month later that more than 150 people had died.

State media has focused on stories of rescuers working to reach the disaster site or reporting on official notices such as President Xi Jinping’s instructions to “make every effort in searching for missing people.”

About 90 Americans were unaccounted for. U.S. President Donald Trump told reporters in the Oval Office Thursday that he offered to provide Nepal's leaders “anything they want.”

Some of the missing may have been on a pilgrimage to Mount Kailash in Tibet, a sacred site for Hindus, Buddhists and others.

Vishnuram Ramaswamy, who works at a private company in the southern Indian city of Puducherry, said that he last spoke to his mother and two sisters who were on that pilgrimage Wednesday morning. It was a highly anticipated trip, something his mother had always wanted to do, he said.

“My mother said they were on their way towards the border area and said they’ll reach their hotel and call us,” he said. He's in touch with government officials and the travel agency, but there's been no news.

“This was the last we spoke to them. They have been unreachable since.”

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Associated Press journalists Eileen Ng in Kuala Lumpur, Malaysia; Sibi Arasu in Bengaluru, India; Huizhong Wu in Bangkok; Upendra Mansingh in Nuwakot district, Nepal; Aijaz Hussain in Srinagar, India; Rod McGuirk in Melbourne, Australia; Ali Swenson and Edith Lederer in New York; Matthew Lee, Collin Binkley and Fu Ting in Washington; and Hannah Schoenbaum in Utah contributed to this report.