The death toll from catastrophic flash floods and mudslides in Nepal rose to at least 626 on Saturday, as more than 2,400 people remained missing in the country, including around 90 Americans.

Rescuers have struggled to reach devastated communities along the Nepal-Tibet border, with the scale of the disaster coming into view four days after the torrent of water, ice and debris swept through Himalayan valleys, destroying homes, bridges and other infrastructure.

As families wait for news about their relatives, damaged roads, power outages and fears of further flooding have hampered rescue efforts.

China has separately reported seven deaths and 554 people missing in its Tibet region, including hundreds of foreigners. It is unclear how much overlap there is between the Chinese and Nepali missing-person counts.

The disaster began Wednesday when a massive chunk of ice and rock dozens of times larger than the Empire State Building broke off a mountaintop and triggered the catastrophic flooding, geologists said.

Videos from the hardest-hit border regions showed people running as the debris flow swallowed buildings and vehicles, devastated towns and settlements, and severed roads, power and communications. The suspected collapse of a melting glacier shook the ground so violently that seismic monitors initially registered it as a magnitude-4.4 earthquake.

The State Department said it is not aware of any confirmed American deaths, but the large number of U.S. citizens still unaccounted for has added to the growing uncertainty surrounding the disaster.

Among the missing are Murphy, Texas residents Raghu Rajan and Archana Suresh. The pair were expected to cross the China-Nepal border Wednesday morning when the floods hit. Their phones last pinged that morning near a bridge that washed away.

“We know the severity of it, but still, you know, there’s a ray of hope for us to find our loved ones,” Rajan’s sister-in-law Divya Chandrashekar told NBC Dallas-Fort Worth.

Deepak and Madhu Ahuja, Plano, Texas business owners who have been married for 34 years, were in the area for a religious journey to Mount Kailash, a sacred site for both Hindus and Buddhists. Their family is now desperately trying to find them while holding out hope that they survived.

“My first reaction was, ‘That cannot be true,’” Deepak’s brother Sushil Ahuja said. “They went there for a good cause, right? This cannot happen to them.”

The State Department has urged U.S. citizens “currently in, or recently traveling through Rasuwa or another affected area” to get in touch via KathmanduACS@state.gov.

A spokesperson said relatives seeking information about family members in Nepal “should call 1-888-407-4747 toll-free from the United States and Canada, or 1-202-501-4444 from other locations.”

On Saturday, helicopters resumed operations in Nepal’s flood-hit Nuwakot district after poor weather temporarily halted efforts.

Some helicopters arrived loaded with boxes and bags of aid, including clothes, instant noodles, crackers and rice, while others returned from the flood-hit areas carrying survivors, among them several babies from the nearby community of Trishuli.

Scores of bodies recovered along the banks of the Trishuli and Narayani rivers have been brought to the city of Pokhara on Phewa Lake in central Nepal for identification.

At the Western Regional Hospital in Pokhara, a crowd of families gathered Friday outside the facility’s morgue as they anxiously waited to see if their loved ones might be among the dead being brought to the medical facility.

Inside the hospital, a digital display could be seen being set up to show photographs of the faces of some of the victims, in hopes that their loved ones might be able to identify them.

The display was set up by police, with authorities saying some of the bodies recovered from the floods were in an advanced state of decomposition, making physical identification difficult.

Meanwhile, rescuers are working “day and night” to search for survivors amid challenging conditions, a U.N. official said.

“The government has established a relief fund, and already the World Bank, India, China and the U.S. have committed to pledge $229 million to support,” Lila Pieters Yahia, the U.N. resident coordinator in Nepal, said, adding that the U.N. is also providing $2 million from its humanitarian relief fund for immediate assistance.

Nepal may need $4 billion to $5 billion to rebuild following the disaster, the country’s finance minister Swarnim Wagle told Reuters on Saturday.