KATHMANDU – Nepal sent helicopters to the rescue on Wednesday after a massive flash flood swept away villages in the Himalayan nation, killing eight and damaging roads, bridges and power projects in areas near Tibet, authorities said.

Across the border, Tibet reported “major casualties” with some missing in the neighboring county of Gyirong after a mudslide hit a land crossing between China and Nepal, the Xinhua official news agency said.

The waters were too high for helicopters to land in Nepal’s affected areas of Syapru Besi and Timure in the mountainous district of Rasuwa, which has a population of about 50,000, officials said, while flagging the risk of casualties.

“There could be heavy casualties or loss of property,” said district administrator Narendra Pariyar. “But I cannot say for sure now.“?

Pariyar urged those living on the banks of the Bhote Koshi river in the area to seek higher ground, pointing to reports that nearby villages and infrastructure project sites were washed away.

“It all came suddenly,” the Onlinekhabar portal quoted a witness from the area as saying. “We heard a loud sound, like a volcano erupting. People started to scream and ran away.”

Nepal’s rescue helicopters would be unable to land in the affected areas until the waters recede, however, said army spokesperson Raja Ram Basnet.

The cause of the inundation was not immediately clear, but a deadly flash flood on the same river last year was eventually traced to the draining of a glacial lake.

The mudslide on the Nepali side hit China’s Gyirong port, Xinhua added, cutting off roads, communications and power in the Shigatse region near the border with Nepal, it added without giving details.

Chinese President Xi Jinping urged “all-out” search efforts for the missing as well as stronger monitoring and early warning systems to prevent secondary disasters.

Nepal’s energy ministry said 430 megawatts of electricity supplies were affected, mostly from hydroelectric projects, or more than 12% of total national hydropower capacity of 3.2 gigawatts, a calculation showed.

Nepali Prime Minister Balendra Shah said police and military were joining rescue and relief teams.

“Instructions have been given ... to move the people living in the lower areas to safer places,” he said. “People along the river are advised to exercise extra caution and remain alert.”

From its origins in Tibet, the Bhote Koshi river empties into Nepal’s Trishuli river that eventually flows into India as the Gandak river.

Last year’s Bhote Koshi flood killed nine people, with 24 missing, as it washed away a “Friendship Bridge” linking Nepal and China to disrupt trade and transport for months.

That flood was triggered by the draining of a supraglacial lake in Tibet, a regional climate monitor said.