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 have now killed more than 500 people, with hundreds still missing.

Nepal’s National Disaster Risk Reduction and Management Authority says the death toll in flash floods has risen to 538.

Nepali police later put the number of deaths higher at 547.

The authority said in an update Friday that the number of missing people remained at 977 while 3,742 people have been rescued.

The authority said 14,829 security personnel have been mobilized for search and rescue operations.

China state broadcaster CCTV said Friday that five people died in flooding along Tibet’s border with Nepal and 558 remained missing. Tibet is a region in China.

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(MORE: How did the deadly flood disaster in Nepal and Tibet happen?)

Authorities warn of further flooding from barrier lake

Nepal police issued a fresh alert Friday after receiving information that a dam on the Tibet side has 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 is unclear if the dam is related to the flooding concern from a barrier lake that formed after the initial flash floods, high up in the Chinese side of the Himalayan mountains.

(MORE: Manhattan-sized iceberg snaps off glacier in Greenland)

Nepal police spokesperson Abi Narayan Kafle told The Associated Press that the rescue operation was continuing but with high alert. “It hasn’t stopped.”

Nepali and Chinese authorities earlier warned of further flooding from the barrier lake that formed after Wednesday’s disaster and was already overflowing. They urged people in potentially affected areas to move to safer ground.

Although Chinese state broadcaster CCTV reported Friday morning that the lake’s barrier was more than 10 kilometers (6 miles) from Gyirong Port, the hard-hit border crossing with Nepal, it said that the risk of flooding was high.

CCTV said the lake was at an elevation of 2,950 meters (9,680 feet), about 1,000 meters (3,280 feet) above Gyirong Port.

Thousands of people have been evacuated; others await rescue

Nepali Army rescuers were working to help people trapped in a hydropower tunnel in Rasuwa, part of the Upper Trishuli-1 Hydropower Project.

In video from the scene, rescuers half-pulled, half-pushed a man covered in mud out of the tunnel. He was one of likely dozens trapped in the tunnel.

(WATCH: Watch as raging floodwaters snap bridge in half in Nepal)

Army spokesperson Brig. Gen. Raja Ram Basnet told 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’ve 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, it reached a magnitude 5.2 seismic measurement. 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.

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

“Right after the shake, I noticed a black storm coming toward 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.”

He described Timure as a vibrant place filled with government offices, hotels, cafes, banks, primary schools, tea shops and residential buildings, where more than 1,500 people were living.

(MORE: A looming rock collapse threatens a Swiss village as thawing permafrost destabilizes the Alps)

Families around the world hear the news of loved ones

Rajan Tamang, a restaurant owner in New York City, said he learned from family in Kathmandu that his mother and sister were among those who died, while many other relatives and friends remained missing.

“My family was just waking up, and they didn’t even get any information, and suddenly this flood came. Some of them are missing, some of them lost their life and only a few is alive,” he told AP after a vigil in New York.

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 pilgrimage to Mount Kailash on Wednesday morning at 9 a.m.

“My mother said they were on their way towards the border area and said they’ll reach their hotel and call us,” he said. “This was the last we spoke to them. They have been unreachable since.”

Ramaswamy said he’s in constant contact with Nepal and Indian government officials as well as the travel agencies who arranged his family’s trip but has not received any updates about their whereabouts so far.

“This was the first time they had gone on this pilgrimage and it was something my mother always wanted to do,” he said.

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

About 90 Americans were unaccounted for. U.S. President Donald Trump said the flooding was terrible to see, and he told reporters in the Oval Office he offered to provide Nepalese leaders “anything they want.”