A mudslide originating on Nepal's side of the China-Nepal border struck the area around Gyirong Port in southwest China's Xizang Autonomous Region on August 26, leaving heavy casualties and a large number of people unaccounted for and prompting a large-scale rescue operation.

Here is what is known so far about the disaster, its causes and the ongoing response.

What happened?

At around 10:30 a.m. on August 26, a mudslide on Nepal's side of the border caused heavy casualties and left a large number of people missing at Gyirong Port in Gyirong County, Xizang, according to local authorities.

As of the latest figures released by local authorities on Thursday, three people had been confirmed dead and 558 remained unaccounted for, while two local villagers had been rescued. Among those unaccounted for were 260 foreign nationals.

The disaster disrupted roads, communications and power supplies in the area. Authorities have evacuated residents from villages deemed at risk, while efforts are underway to restore communications and clear damaged roads.

The Chinese Foreign Ministry said Thursday that nearly 100 Chinese nationals remained unaccounted for in the affected area in Nepal. The Chinese Embassy in Nepal is verifying the information with local authorities.

The death toll from devastating floods has risen to 359 in Nepal, as search and rescue operations continue for 910 people who are still missing, according to local authorities.

What caused the disaster?

According to experts cited by Chinese authorities, the disaster was triggered by a high-elevation ice avalanche in Nepal.

The Ministry of Natural Resources said the ice avalanche generated a high-speed debris flow, which then evolved into a mudslide as it moved downstream, carrying ice, rocks and other debris.

The terrain also amplified the disaster's intensity. According to Xu Qiang, a geological disaster expert, president of Chengdu University of Technology and chief scientist at the National Key Laboratory of Geohazard Prevention and Geoenvironment Protection, the elevation difference between the ice avalanche source area and Gyirong Port is several thousand meters. The source area is estimated to be around 5,000 meters above sea level, while Gyirong Port sits at about 1,830 meters above sea level.

Xu said the steep terrain allowed the debris flow to travel at extremely high speed. He added that the volume of material involved was massive, potentially reaching tens of millions of cubic meters.

Another factor was the high exposure of the area. Gyirong Port is located at the confluence of three rivers, with buildings and facilities distributed along the flow path.

Why is the barrier lake being closely monitored?

The mudslide also caused an ice-rock debris blockage at a river confluence, forming a barrier lake.

According to China's Ministry of Water Resources, the lake held an estimated 2 million cubic meters of water as of Thursday morning and had already begun overtopping. An additional 3 million cubic meters of water is expected to flow into the lake over the following three days, the ministry said.

The ministry said it is closely monitoring changes to the barrier lake and carrying out calculations on potential flooding in the event of a breach to support rescue and emergency response efforts.

Experts have also warned that the blockage and ongoing rainfall could lead to further water accumulation upstream. Rescuers have therefore been advised to monitor upstream water flows and guard against possible secondary disasters.

How is the rescue operation progressing?

Multiple government departments and rescue forces have been deployed to the disaster area.

Xizang emergency management authorities have dispatched 152 vehicles and 792 personnel, while the regional fire and rescue corps has sent 72 vehicles and 431 rescuers equipped with search dogs, life detectors and other specialized equipment.

On Thursday morning, the first 141-member rescue team dispatched by China's Ministry of Emergency Management reached Gyirong Port. Rescue personnel are conducting search and rescue operations while gathering data on the upstream blockage.

China Anneng, a state-owned engineering and rescue enterprise, sent an advance team equipped with communications and detection equipment to assess the upstream blockage. According to Xinhua, addressing the blockage is a priority before accumulated debris roughly 3 kilometers upstream of the port can be safely cleared.

Concrete progress has been made in infrastructure restoration and personnel relocation. As of 4 p.m. Thursday, communication signals had been restored at a point 3 kilometers from the Gyirong Port border crossing, marking phased progress in emergency communication support. By 6 p.m. the same day, around 800 meters of the damaged section of National Highway G216 leading to Gyirong Port had been cleared and reopened.

A total of 499 affected residents have been properly relocated, with more than 320 people moved out of high-risk areas. With the assistance of cultural and tourism authorities, 555 tourists in Gyirong Town have been evacuated, and no tourists remain stranded in the disaster area.

Meanwhile, meteorological authorities have forecast rainfall and strong convective weather in the Gyirong area and toward Nepal's side of the border over the next three days. Experts have warned of possible secondary disasters, including further mudslides and landslides.

Read more: Graphics: Updates on rescue operations for mudslide at Gyirong, Xizang

(With input from agencies)