At least 17 people have been killed and nearly 400 others are unaccounted for after a flash flood and mudslide along the Nepal-Tibet border washed away villages and damaged roads and infrastructure.

The flood hit early on Wednesday in Rasuwa, just north of Nepal's capital Kathmandu, with preliminary information suggesting it was caused by an earthquake triggering an avalanche with melted snow pouring into the Bhote Koshi river.

Search operations are under way on both sides of the border between Tibet and Nepal and 12 people have been rescued so far.

State media in China, which controls Tibet, say there are significant casualties and Nepalese officials said they anticipate "huge scale loss".

The Nepal tourism board said work was under way to establish the whereabouts of 291 tourists and 93 Nepalese nationals, including guides, who were known to be in the area where the flood hit.

They include 12 travellers from the UK as well as Americans, Malaysians, South Africans and Indians.

Videos show floodwater crashing through villages. Nepal Police spokesman Abhi Narayan Kaphle told the BBC that all security agencies were working together on rescue efforts.

Several areas downstream of the Bhote Koshi river, which flows from Tibet to Nepal, are on high alert and roads, communications and power in the region have been disrupted. The authorities say the main road to Kathmandu is closed.

"Our logistics company has seven container trucks there, all staffed by Nepalese employees. We have lost contact with all of them. There is no signal, so we cannot reach anyone," Yee Liang, a Chinese businessman based in Kathmandu, told the BBC.

"As far as I know, there was no warning. There were occasional mudslides and landslides, especially in the rainy season, but this one seems much more serious."

The extent of the damage on the Tibetan side is unclear but the BBC spoke to Guan Daoxuan in Sichuan province who said he had lost contact with his wife late on Wednesday morning.

She works for a state-run construction company involved in building roads and bridges in Tibet. Guan said Nepalese authorities had tried to send a helicopter to inspect the area because road access had been cut off, but strong winds were making it difficult for the aircraft to reach the site.

Nepal's disaster management authority called on people living near river banks in vulnerable areas to remain alert and take precautions.

"Work has started to take necessary steps in co-ordination with the medical team, scouts, and Red Cross," Prime Minister Balendra Shah said in a social media post, after holding an emergency meeting to take stock of the situation.

Nepal's Foreign Affairs minister Shishir Khanal told parliament that 26 police personnel, nine armed police officers and 44 members of the military were missing.

He said an earthquake at 08:37 local time had triggered an avalanche and there was preliminary information that this had resulted in the flash flood.

Rasuwa saw deadly flooding last year that killed 18 people after a glacier burst on the Chinese side of the border.

Officials have told the BBC that they believe the scale of Wednesday's disaster exceeds last year's floods.