Three British nationals are missing after a huge earthquake in Turkey and Syria.
It comes as UK aid charities say reports of the devastation are just the “tip of the iceberg”.
Foreign Secretary James Cleverley said the department’s Crisis Response Hub is working to support at least 35 Britons caught up in the disaster.
He added: “We assess that the likelihood of large-scale British casualties remains low.”
A number of relief organisations have urged the public to dig deep and donate, saying the help they are able to provide over the next few days “will save lives”.
More than 5,000 people across both nations are so far confirmed to have died in the natural disaster, though the figure is rising as a search and rescue operation continues.
Difficult conditions, including freezing temperatures, are hampering efforts, particularly in rebel-held Syria, where people have fewer resources and there is a lack of routes to deliver aid through.
Among those joining the relief effort are 12 crew members from the London Fire Brigade, while a plane carrying 77 UK search and rescue specialists, state-of-the-art equipment and four search dogs landed in Gaziantep on Tuesday evening in response to a request from the Turkish government.
The team, made up of firefighters and staff from 14 fire and rescue services from across the country, will cut their way into buildings and help locate survivors in the rubble of the earthquakes.
Their specialist equipment includes seismic listening devices, concrete cutting and breaking equipment and propping and shoring tools.
An emergency medical team was also on the flight and will conduct a full assessment of the situation on the ground.
Rishi Sunak “pledged the UK’s steadfast support” to Turkish president Recep Tayyip Erdogan during a phone call on Tuesday afternoon, Downing Street said: “He expressed his deep condolences for the tragic loss of life and pledged the UK’s steadfast support.”
No 10 said: “The Prime Minister also noted the deeply concerning humanitarian situation over the border in north-west Syria, where Turkiye (Turkey) plays an important co-ordinating role, and set out how the UK has increased support to aid organisations and emergency responders.”
The King told President Erdogan his “thoughts and special prayers” are with all those affected by the earthquakes.
He said: “My wife and I have been most shocked and profoundly saddened by the news of the devastating earthquakes in South East Turkiye (Turkey).
“I can only begin to imagine the scale of suffering and loss as a result of these dreadful tragedies and I particularly wanted to convey our deepest and most heartfelt sympathy to the families of all those who have lost their loved ones.
“Our thoughts and special prayers are with everyone who has been affected by this appalling natural disaster, whether through injury or the destruction of their property, and also with the emergency services and those assisting in the rescue efforts.”
British Turkish Association spokesman Atilla Ustun, 55, praised communities across London, which have helped raise up to £300,000, which has paid for 300 boxes of donated aid to be sent on a Turkish Airlines cargo plane from Heathrow.
Dilan Altun, a 22-year-old Turk living in London, said she has tens of relatives who are now homeless and has been told people are dying after being rescued due to sub-zero conditions.
After the rescue mission, providing shelter is aid organisations’ priority, while there is also a need for food, clean water and warm clothes.
James Denselow, UK head of conflict and humanitarian advocacy for Save the Children UK, told the PA news agency: “The scale of this earthquake, in terms of not just strength but the kind of actual absolute sprawl of it has meant that we’ve had to spend a lot of time in this first phase checking in on needs, checking in on what is working logistically, checking that all our people are OK.
“Because you’ve got airports out of action, hospitals collapsed, clinics collapsed, all the sort of places we would normally use are not necessarily accessible.”
He added: “Providing shelter is the most urgent type of aid from our perspective because the cold will kill people in ways that are less spectacular than the earthquake but equally deadly.”
Mr Denselow said a historical aid presence in both countries will help get aid to people in need quickly but the situation is more complicated in conflict-torn northern Syria.
“If you’re a vulnerable population and then something else like this happens, obviously what happens to you is likely going to be far worse.
“We see that with very basic things like children’s physiology. The ability of a child to survive crash injury from a building falling on them is far reduced if they are malnourished.”
Mike Noyes, humanitarian director at ActionAid UK, said it has deployed workers from its team in Jordan to the disaster zones and committed £40,000 from its emergency reserves to start giving relief.
“Right now we’re trying to make sure we have the funding for this support because the financial needs are going to be massive in the immediate and longer term as we help people recover and rebuild,” Mr Noyes said.
He said teams the charity works with in Syria have been personally affected by the disaster.
“They’ve had injuries and deaths within their team and their families. But they’re doing their best at the moment to work with the communities they’re in touch with inside Syria,” he said.
He added: “We know there are huge pressures on people in this country at the moment. We also know from experience that people in this country open their eyes and see what’s going on in the rest of the world and have a great deal of sympathy.
“We’re confident in the UK public’s willingness to be generous. They are always willing to support those in desperate need at a time of humanitarian crisis. They’ve shown it again and again.”
The United Nations this year launched a record 51.5 billion US dollar (£43 billion) appeal to help 339 million people now in need of assistance in 69 countries and 100 million who are displaced – a result of, among other issues, the pandemic on the world’s poorest, droughts and flooding in East Africa and Pakistan and the Ukraine conflict.