Thirty-three people from the UK, including a 13-year-old girl and 14-year-old boy, are among hundreds missing following a deadly flash flood near the Nepal-Tibet border.

Nepali police said the death toll had risen to 359, while Nepal's National Disaster Risk Reduction and Management Authority said more than 1,300 people are missing,with the Nepal Tourism Board saying 644 tourists are out of contact.

Prime Minister Andy Burnham said the UK has offered £5m to help the Nepalese government following the floods, saying it was a "very distressing and devastating situation".

The King said he and the Queen were "heartbroken" and sent their "most profound sympathy" to those affected.

Burnham had previously urged Britons in the region "to follow guidance from the local authorities and follow our travel advice".

The UK Foreign Office said it is working closely with Nepalese authorities "following concerning reports of British nationals who are missing" and has encouraged them to follow local guidance and "remain alert to the changing situation".

Excursion company Alpine Eco Trek, which offers guided expeditions and tours in Nepal, confirmed to the BBC that it has not been able to establish contact with a group of 12 Britons who were booked on a tour to Mount Kailash, and were in the region when the flash floods struck.

According to the company's travel logs, shared with the Press Association, the age of the missing British citizens ranges from 13 to 65.

It said the youngest member of the group was a 13-year-old girl.

The company said it had made attempts to charter a rescue helicopter but were unable to fly due to poor conditions, and will continue to help with ongoing rescue efforts on Thursday.

It also reported four of the firm's Nepali tour guides are also missing.

The company's manager Kumar Adhikari said he was in contact with the guides until 08:40 when they were expected to be at Tibet's immigration office, but their phones were disconnected.

"We were hoping that inside it was safe," he told PA, "but later the video came from China's government which [showed] even Tibet's side was hit by the flood."

Adhikari said he was "slowly responding to" the families of the Britons but has been unable to give them any updates, adding he has been in contact with the British consulate and forwarded them copies of the clients' passports.

"I feel so sad. We are just thinking about what to do next...We are in pain at the moment, deep pain."

Other tour companies have also been impacted by the flash floods, with British citizens among other tours being affected.

The Trekkers' Society, which organises tours in Nepal, Tibet, and Bhutan, said nine UK nationals on a pilgrimage to Mount Kailash are missing after they crossed the Nepal-Tibet border.

It said four males, aged between 14 and 50, and five females aged 40 - 53 remained unaccounted for.

The society's general manager said the group were at Chinese immigration at the border when the flood happened, but it has no contact with the tour guide since.

Himalayan Glacier US told the BBC they have lost contact with a group of 47 guests and staff who where making a pilgrimage to Mount Kailash, which is considered the spiritual home of the Hindu god Shiva.

It said the group included twoBritish citizens.

Kailash Journeys also reported a group of 27 guests and staff missing, with a spokesperson saying they were last in contact with them early Wednesday morning local time.

A list of names provided to the BBC included two men who are British citizens.

Fishtail Tours, a Nepali travel agency offering several climbing expeditions and tours of Nepal and Bhutan, said one British person was missing.

Meanwhile, Pramod Pudel who lives in East Sussex but is originally from Nepal, told BBC Radio Sussex that "nothing remains" where his family lives and he is flying back to search for his wife, mum, brother and uncle.

"It's a very, very difficult time for me and for my family and my country at this moment, but obviously I have to go and see what's happening there," he said.

"A lot of people are missing and there is no information regarding who is missing, where they are. Nobody can contact each other and all the bridges are broken, so you can't cross the land.

"There's no power, no electricity, so I can't contact anyone, so it's a very difficult time not knowing what's happening."

On Thursday, UK Foreign Secretary Ed Miliband said he had spoken to his Nepali counterpart, and said the country would do all it could to "help Nepal in its hour of need".

Miliband said he "offered financial, search and rescue and other help to aid the disaster response.

In a joint statement with Australia, the EU, Finland, France, Norway and Switzerland, the UK embassy in Nepal urged British nationals in Nepal to "exercise caution in affected areas".

The flood hit early on Wednesday in Rasuwa, just north of Nepal's capital Kathmandu.

Preliminary information suggested it was caused by an earthquake, but the US Geological Survey said additional analysis indicated "the seismic energy was instead generated by a landslide."

It said a magnitude 5.2 seismic event caused by a "glacial collapse and debris flow" occurred near the border at 08:37 local time on Wednesday.

Dramatic CCTV footage showed a wall of water and mud hitting buildings near a border crossing and video filmed from inside a bus in Trishuli captured dozens of people trying to flee a flood surge that was surging through the valley.

Meanwhile Chinese state media outlet CCTV said a mudslide in Tibet's Gyirong killed three people and left 265 missing, with rescue efforts under way.