Turkish President Recep Tayyip Erdogan visits the disaster zone in his country as criticism grows over the official response
Families in some badly-hit areas have said the slow speed of rescue efforts means they have had no help digging to find relatives
President Erdogan defends the response, saying it's not possible to prepare for disasters of this magnitude
More than 12,000 people in southern Turkey and northern Syria are now known to have been killed in Monday's earthquakes
Erdogan acknowledged there'd been difficulties with the initial response but blamed delays on damaged roads and airports
In Syria, the White Helmet group who are leading efforts to rescue people in rebel-held areas, say time is running out to save people
Dramatic footage and pictures are continuing to emerge of rescues in both countries
Video obtained by BBC News shows the destruction on the ground in a town near the epicentre of the first earthquake.
Footage from Gaziantep, Turkey, captures almost flattened buildings and piles of rubble.
Inspecting the damage near the earthquake epicentre in Gaziantep, Turkey
Fundanur Öztürk
BBC Türkçe, Ankara
Image source, ReutersRescuers look for survivors in a collapsed building in Malatya
In Malatya, Turkey, 25-year-old Ozgul Konakci left a building with her family and saw that five other buildings around them had collapsed, trapping their neighbours under the rubble.
“They are trying to rescue them still, but it is very cold and snowing. We don’t know what to do, we are just waiting," she says.
"Some people wanted to go back to their houses because it was too cold, But then we felt strong aftershocks and they were out again. They could at least get some clothes for our children, too."
She adds: "We were only worried about having some clothes for our children to keep them warm.”
Turkey earthquake: Snow and rain could impact rescue
Rescue efforts in the coming days could be hampered by difficult weather conditions due to set in over some of the areas affected.
In the Turkish region around the epicentre of the first quake, heavy rain is expected, while temperatures will be only 3-4C during the day and well below freezing at night.
Around 3-5cm of snow is also set to fall, while further north in Turkey, heavier snow is expected.
In more mountainous regions, temperatures are not expected to rise above freezing at all in the coming days, which could mean snowfall of 50-100cm.
At least 2,800 buildings are thought to have been destroyed in this morning's first quake, meaning many thousands of people could be left without shelter.
These images show the extent of the damage done to buildings in the area around the epicentre of the first quake in the early hours of Monday - the cities of Iskenderun and Gazantiep.
Image source, .Entire shops and homes have collapsed upon themselves, and the surrounding streets are full of rubble and debris.
Image source, .A young child is pulled from the rubble in Azaz, Syria, following the devastating earthquake that shook Syria and Turkey, leaving more than 1,200 people dead.
This video contains upsetting footage.
A young child is pulled from the earthquake rubble in Azaz, Syria
As we mentioned earlier, a second earthquake, which had a magnitude of 7.5, has struck in the Elbistan district of Turkey's Kahramanmaras province.
Elbistan lies around 80 miles directly north of Gaziantep, where the epicentre of this morning's quake was situated.
The second quake struck at 13:24 local time (10:24 GMT).
An official from Turkey's Disaster and Emergency Management Authority said it was "not an aftershock" and was "independent" from this morning's quake.
At least 70 deaths had already been reported across Kahramanmaras following the first quake.
It's not yet clear what impact this second quake has had after this morning's tremor, which killed more than 1,200 people. We're continuing to get information and will keep you updated.
Image source, ReutersIf you're just joining us, here's a quick round-up of what's going on in southern Turkey and northern Syria.
A 7.8 magnitude earthquake... has killed more than 1,200 people, and injured thousands more, in south-eastern Turkey and north-western Syria after it hit in the early hours of Monday
The epicentre... was near the southern Turkish city of Gaziantep, but the quake was felt across Turkey, Syria, Lebanon, Cyprus, and Israel. Seismologists say the quake is one of the largest ever recorded in Turkey
Another earthquake... has since hit the same region of Turkey, with the epicentre this time being near the city of Kahramanmaras.
Shocking images... show massive destruction to buildings, roads and teams of rescuers searching for people trapped beneath the rubble
In Turkey... schools in 10 cities and provinces are to be shut for a week, while airports in Hatay, Maraş, and Antep have been closed or partially closed
Around the world... leaders from across Asia, Europe, and the US have pledged to send support following an international appeal by Turkey
Image source, .A second earthquake has struck in south-eastern Turkey, with its epicentre near the city of Kahramanmaras, according to reports.
The US Geological Survey said it had measured the strength of the quake to be 7.5 magnitude.
We'll bring you more details on this when we have them.
As we've been reporting, many hundreds of people have also died in Syria, despite the epicentre of this morning's earthquake stemming from neighbouring Turkey.
Videos and images are emerging of widespread destruction.
One video verified by the BBC, external in the north-west city of Aleppo shows residents fleeing and screaming as buildings collapse into an enormous cloud of dust.
Some areas badly affected by the quake are not under government control, so access to medical care and emergency supplies are limited.
The White Helmets, a humanitarian organisation working in rebel-held areas in Syria, have called for urgent aid.
Image source, ReutersWe've just heard from Turkish President Recep Erdogan, who said that at least 912 people had died in Turkey following this morning's earthquake.
The latest estimate we have for Syria, which comes from the UK-based Syrian Observatory for Human Rights, is 320. The earthquake hit near the border between the two countries.
That means the total estimated death toll for the quake now exceeds 1,200.
Erdogan also said that at least 5,383 people had been wounded in Turkey. Dozens more are injured in Syria.
The death tolls in both countries are expected to rise. Stay with us for more live updates.
Turkey's President Recep Erdogan says 912 people have been killed in Turkey alone, and 5,383 wounded, due to the earthquake.
He says he can't predict how much the death toll will rise by as search and rescue efforts continue.
This is the country's largest disaster since 1939, Erdogan tells reporters, adding that 2,818 buildings collapsed as a result.
Pallab Ghosh
Science correspondent
The earthquake has occurred around a region of instability called the East Anatolian fault, which runs south-west to north-west of the south-eastern border of Turkey.
Seismologists have long recognised that this fault is very dangerous, though there has not been any significant activity for more than 100 years, but it has been responsible for very damaging earthquakes in the past.
In particular, on 13 August 1882, when it caused an earthquake registering 7.4 in magnitude, significantly less than the 7.8 magnitude recorded today.
Even so, that 19th century earthquake resulted in immense damage to towns in the area, with 7,000 deaths recorded on the city of Aleppo. Damaging aftershocks continued for nearly a year.
The epicentre of the earthquake that struck Turkey and Syria this morning was near the city of Gaziantep, in south-eastern Turkey.
But people across the country, as well as Syria, Lebanon, Cyprus and Israel, felt the earth move from the 7.8 magnitude quake.
Image source, .Schools in the 10 cities and provinces across Turkey hit by the earthquake will be closed for a week, the country's vice president Fuat Oktay says.
The cities and provinces are are Kahramanmaraş, Hatay, Gaziantep, Osmaniye, Adıyaman, Malatya, Şanlıurfa, Adana, Diyarbakır, Kilis.
Oktay also said flights to and fro the airport in Hatay province have been suspended, while airports in Maraş and Antep are also closed to civilian flights.
Image source, Getty ImagesRescue workers in Gaziantep
The major Turkish city Gaziantep has the highest death toll so far with 80 casualties.
Gaziantep is located about 90km from the Syrian border and historically known as Antep. Several other quakes hit the city on Monday after the initial powerful 7.8 magnitude quake, including one of magnitude 6.4 and another at magnitude 6.5.
Seventy deaths have been reported in Kahramanmaras, with the second-highest death toll.
Turkey's Vice President Fuat Oktay said earlier in the day that 284 people have been killed and 2,323 injured in 10 provinces so far.
Ismail Al Abdullah from the White Helmets, the civil defence organisation best known for working in rebel-held areas of Syria, spoke to the BBC from Sarmada in Syria, near the border with Turkey.
He says: "Many buildings in different cities and villages in northwestern Syria collapsed, destroyed by this earthquake. Our teams responded to all the sites and the buildings - and still now, many families are under the rubble. We are trying to save them but it's a very hard task for us.
"We need help. We need the international community to do something, to help us, to support us. Northwestern Syria is now a disaster area. We need help from everyone to save our people," Al Abdullah adds.
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The president of the Turkish Red Crescent called on the nation to make blood donations.
Kerem Kınık also said on Twitter the organisation sending additional shipment of blood to the affected region.
He also urged the public to leave damaged buildings and clear the streets for rescuers.
As daylight broke, the scale of the devastation across Turkey and Syria became clear.
Image source, Getty ImagesSearch and rescue operations take place in Turkey's Kahramanmaras province
Image source, EPAEmergency personnel search for victims at site of collapsed building in Diyarbakir, Turkey
Image source, ReutersPeople search through rubble in Diyarbakir, Turkey
Image source, ReutersA rescue worker carrying a child in the rebel-held Syrian city Azaz
Image source, EPASearch continues for victims at site of collapsed building in Diyarbakir, Turkey
Rescue teams are searching for survivors after a devastating earthquake shook Turkey and Syria in the early hours of Monday.
Residents in the Turkish city of Diyarbakir describe what they have witnessed.
Residents in Diyarbakir describe the scene