A 7.2-magnitude earthquake has hit eastern Turkey, killing at least 45 people in one town alone as buildings collapsed, Turkish media say.
The quake struck near the city of Van, where Anatolia news agency said at least 50 people were injured.
Prime Minister Recep Tayyip Erdogan's office said there had been damage and deaths in Van but gave no firm figure.
Turkey is particularly vulnerable to earthquakes because it sits on major geological fault lines.
Two earthquakes in 1999 with a magnitude of more than 7 killed almost 20,000 people in densely populated parts of the north-west of the country.
'We need medics'
The earthquake struck at 10:41 GMT (13:41 local time) with its epicentre 16km (10 miles) north-east of Van, the US Geological Survey said.
It was followed by a series of powerful aftershocks, also centred north of Van, including two of magnitude 5.6.
The main quake was at a depth of 20km, the USGS said.
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People are helped from the rubble of collapsed buildings in a village near the city of Van, Turkey - 23 October 2011
In pictures: Turkish earthquake
Turkish television said 45 people had been killed and 150 injured in the town of Ercis - some 60km north of Van.
Deputy Prime Minister Besir Atalay said 25-30 buildings had collapsed in Ercis and 10 had collapsed in Van.
According to the Turkish Red Crescent, 25 buildings containing flats and one housing a dormitory had collapsed in Ercis, AP said.
Zulfikar Arapoglu, the mayor of Ercis, told NTV: "There are so many dead. Several buildings have collapsed, there is too much destruction."
"We need urgent aid, we need medics," he is reported by the Associated Press news agency as saying.
'Voices'
Television pictures from Van showed damaged buildings and vehicles crushed by masonry, with panicked residents spilling into the streets.
Local official Veysel Keser said: "Many multi-floor buildings, hotels and a dormitory were collapsed."
Map
"We can hear voices from the collapsed buildings," AFP news agency quoted him as saying.
Rescue workers are scrambling to remove people from the rubble, before nightfall brings temperatures down close to 0C.
Initial reports of damage and casualties have been hard to come by, with the earthquake cutting telephone and electricity lines.
The head of Turkey's seismology institute said hundreds of people may have been killed.
"We estimate around 1,000 buildings are damaged and our estimate is for hundreds of lives lost. It could be 500 or 1,000," said Mustafa Erdik, the general manager of the Kandilli Observatory.
The BBC's David O'Byrne, in Istanbul, said more search and rescue teams were being sent from other parts of the country.
Hakki Erskoy, from the Turkish Red Crescent, said aid teams from the north and east of Turkey were being sent to the earthquake-hit area.
He said camps were being set up to shelter people and blankets, food and water were being sent along with mobile kitchens.
Military aircraft were being deployed to help with the rescue and relief efforts, Mr Erskoy told BBC World News.
