More than 1,400 people are unaccounted for after the disaster, as the death toll surged to at least 362, with entire villages swept away.

Beijing's foreign ministry said nearly 100 Chinese nationals were among the missing in Nepal.

That was in addition to the hundreds missing, including 260 foreigners, on its side of the border.

Chinese Premier Li Qiang arrived at the mudslide site in Tibet on Thursday afternoon to "guide emergency rescue and relief work", state media reported.

A wall of mud and debris swept over buildings on the Chinese side of the border on Wednesday, with surveillance footage showing dozens of people running as the surge crashed down.

"There was a rumbling sound," a Chinese businessman surnamed Deng told AFP as he recounted visiting the border area Wednesday morning.

He said he and a friend were planning to buy Nepali cigarettes sold at the checkpoint, but "my friend changed his mind and we turned back halfway".

"If we had gone down there, we wouldn't have made it back," Deng said.

He was around 10 kilometres from border when he heard rumbling.

"After it happened, we just felt incredibly lucky," he said. "We never imagined such a massive mudslide could happen."

Rescuers are facing multiple hurdles, including "the instability of a reservoir lake upstream" and "the narrow mountain road", Li Qipu, a member of China's paramilitary People's Armed Police, told state broadcaster CCTV.

Authorities have dispatched nearly 800 people and more than 150 vehicles to the border area, according to state news agency Xinhua.

More than 300 villagers near the border have been evacuated, it added, citing authorities in Tibet's Gyirong County where the mudslide hit.

The fire service has deployed 431 personnel, working with search dogs, life-detection scanners and small boats, it said.

CCTV broadcast images of People's Armed Police members in camouflage and orange vests with shovels, and rescuers flying drones.

Civilian and military aircraft carrying relief supplies had also arrived in Tibet, CCTV said.

Overnight rainfall heightened the risk of secondary disasters such as landslides and rockfalls.

The Tibet Autonomous Region is largely inaccessible to foreign journalists, who are generally only allowed to visit on government-organised trips.