At least 150 people have been killed and hundreds of tourists are missing after a massive flash flood and mudslide struck the Nepal-Tibet border on August 26.

The area stricken area is known to be popular with pilgrims and trekkers. Nepal’s tourist board said more than 500 tourists were currently missing in the country, including more than 100 from India, 54 from the US, 33 from the UK and more than 100 from Nepal itself, the BBC reports.

In Tibet, Chinese government media, however, reported three deaths and 265 people missing with CCTV footage showing a wall of water and mud hitting buildings at the Gyirong border crossing.

Dozens of Nepalese police and army personnel are also reported missing, along with power workers. The scale of the disaster is expected to become clearer as rescue teams reach affected areas in the days ahead.

The flooding caused widespread damage to homes, roads and other infrastructure. Video footage shows thick brown water surging through the landscape and destroying bridges. At the border crossing, people were seen fleeing before floodwater hit the building.

Authorities initially believed an earthquake may have preceded the flooding although reports say the US Geological Survey now says the seismic energy was generated by a landslide, rather than an earthquake triggering the disaster.

Nepal’s Prime Minister Balendra Shah said the country has been shocked and deeply distressed by the disaster.

One woman told the BBC that her family had been swept away in front of her. She and her son survived by climbing onto a rooftop.

The Australian government said a number of Australians had been in the flood area but had not confirmed how many were among those reported missing.

Meanwhile, a trekking company operating tours across the Nepal-Tibet border has also been trying to establish the whereabouts of missing customers. The manager told the Press Association that around a dozen Britons from one excursion were missing and that the company had received thousands of calls from their families. Four Nepalese guides working for the company were also missing.

International governments have offered support. India’s Prime Minister Narendra Modi said the country was prepared to provide humanitarian assistance to Nepal. The US State Department said it was in contact with the Nepalese government and ready to assist.

European Commission President Ursula von der Leyen has also said the EU was ready to mobilise further assistance. UK Foreign Secretary Ed Miliband too said Britain’s consular team was working with local authorities on the emergency response and supporting those affected.