The UK has offered £5 million to help the Nepalese government amid a “very distressing and devastating situation” following the country’s flash floods, the Prime Minister has said.

More than 30 British nationals are among hundreds of people missing and feared dead as rescue efforts continued on Thursday.

Andy Burnham told reporters an offer had also been made for emergency responders to head to the affected area.

Nepalese authorities said at least 270 people have died in the flooding in the Rasuwa district, which borders China’s autonomous region of Tibet, where three more have been confirmed dead.

Two teenagers, a 13-year-old girl and a 14-year-old boy, are among the 33 British nationals believed to be unaccounted for, as the total number of missing people in Nepal rose from 403 to 826.

Videos posted on social media appeared to show huge floodwaters surging through valleys as people screamed in terror, with bridges being washed away.

Other videos showed people desperately fleeing uphill to escape the surging muddy floodwaters.

Speaking to journalists during a visit to South Wales, the Prime Minister said: “It’s a devastating picture that is emerging from what has happened in Nepal.

“Scenes that we’ve never seen the like before, I don’t think, certainly the scale of the devastation.

“So there is immediate action being taken. We are deploying consular support staff to the region so that British nationals will be able to access support 24/7.

“An offer has been made of emergency responders to head to the region but that’s obviously a matter for the Nepalese government.

“But also we’ve offered £5 million to help with that response effort.

“Obviously, there are a number of British nationals who currently are thought to be missing and we’ll be providing all the support we can to their families in what is a very distressing and devastating situation.”

A man from Uckfield, East Sussex, said he is flying to Nepal at 8pm on Thursday to try to make contact with his wife, parents, brother and uncle who have been caught up in the flash flooding.

Speaking to the BBC, Pramod Poudel said: “There is no proper communication so I can’t communicate with them.

“I guess there is no power so they can’t charge their phones – so be honest, it’s a very hard and difficult time at the moment.”

In an emotional interview, Mr Poudel added: “I have seen pictures of where my home town used to be… and there is nothing there now.

“Where my family used to live – my mum, my dad, my wife, where everyone used to live – there’s nothing there.”

The president of the Nepal Centre for Disaster Management described the scene as being like a “tsunami from the sky” and that villages have been “completely wiped out”.

Bishal Nath Upreti told BBC Radio 4’s Today programme the damage to the infrastructure has been so extensive only helicopters can be used to reach the site of the disaster.

Nepalese officials said bridges have been destroyed and nearly 40km (25 miles) of roads have been damaged, which has complicated search efforts.

A number of those missing had been making a pilgrimage to the sacred Hindu site of Mount Kailash, Nepalese tour operator The Trekkers’ Society said.

The operator said nine British nationals on one of its tours to the site were among those unaccounted for – four males, aged 14, 50, 50, and 54, and five females, aged 40, 46, 50, 50, and 53.

General manager Roshan Basnet told the Press Association: “When the flood happened, they were still at the Chinese immigration (at) the border. After the flood happened, we had no contact with our tour guide.”

He added: “We have yet to get the confirmation of what state they’re in, but as for the video that is circulating on the internet, we believe that it’s a pretty difficult (situation).”

Travel logs obtained by PA show three men aged 65, 64, and 52, and seven women aged 44, 47, 51, 60, 63, 63, and 65, are also among the missing British nationals.

Tour operator Kailash Journeys confirmed it had not been able to account for seven UK nationals – five women and two men aged between 18 and 55 – travelling with the company.

According to the Nepal Tourism Board, 579 travellers are missing in the affected area in the country, including 466 foreign nationals.

China’s state-run Xinhua News Agency said 558 people remained unaccounted for in neighbouring Tibet.

The United States Geological Survey (USGS) said a magnitude 5.2 seismic event occurred near the border between Nepal and China on Wednesday at 8.37am local time.

It added that a “glacial collapse and debris flow” caused “subsequent catastrophic impacts downstream”.

Sunil Sharma, a tourism board spokesman, said 133 of those missing were from India on pilgrimage to Mount Kailash.

Also among the missing were 47 people from the US, 34 from Australia, and 24 from Canada, he added.

In its analysis, the USGS said: “This event was initially reported as a magnitude 4.4 earthquake.

“Additional analysis of nearby seismic stations, long period seismic waves, and satellite imagery led to the determination that the seismic event was instead a glacial collapse and debris flow, and that no earthquake had occurred.”

The Foreign, Commonwealth and Development Office said it was “deeply saddened by the loss of life and devastation caused by the floods in Nepal”.

It added: “British nationals in Nepal should follow local authority advice and exercise caution while travelling.

“For consular assistance call (+44) (0)207 008 5000 or (+977) (0)14237100.”