Hundreds of aftershocks have rocked devastated areas of central Italy, hampering search efforts after a deadly earthquake.
A strong tremor with magnitude 4.3 struck on Thursday afternoon, sending rescuers fleeing from already fragile buildings.
About 5,000 rescue workers are combing through rubble for survivors using heavy machinery or bare hands.
At least 250 people are now known to have died after Wednesday's quake.
The 6.2-magnitude quake hit at 03:36 (01:36 GMT), 100km (65 miles) north-east of Rome in mountainous central Italy.
More than 300 people have been treated in hospital and dozens are believed to be trapped under rubble.
Worst affected are the towns of Amatrice, Arquata, Accumoli and Pescara del Tronto.
The towns are usually sparsely populated but have been swelled by tourists visiting for summer, making estimates for the precise number missing difficult.
At least three Britons died in the quake, a local official in Amatrice told the BBC.
The hunt for survivors continued throughout Wednesday night and into Thursday. With many buildings unsafe, some people were forced to sleep in their cars or in tents.
Search teams have asked locals to disable their wi-fi passwords to help efforts.
The Italian Red Cross says residents' home networks can assist with communications during the search for survivors.
Rescuers have advised journalists and bystanders to leave Amatrice urgently, as "the town is crumbling", the BBC's Jenny Hill says.
At the scene: BBC's Damian Grammaticas in Pescara del Tronto
Two firemen burrowed deep into the rubble looking for a survivor. "It's a dog," one of them shouted out.
For half an hour the men kept digging. They passed water down to be given to the animal. And eventually they worked it free, then emerged, carrying it to the surface. There was a ripple of applause in the crowd.
"It doesn't matter to us if it's a person or an animal, we save it," said Gianni Macerata, the fire officer in charge.
So the digging goes on. But so little is left of Pescara del Tronto it is unlikely that more survivors will be found here.
It seems unlikely too that this ancient little place, that has stood for centuries, can ever be rebuilt. Hundreds of years of history ended in an instant.
In another hard-hit village, Pescara del Tronto, a young girl was pulled alive from the rubble late on Wednesday after being trapped for 17 hours.
Rescuers say they still hope to find survivors, noting that one was pulled from ruins in L'Aquila in 2009 three days after an earthquake that killed more than 300 people.
But an official with the fire department, Lorenzo Botti, admitted they were facing a race against time.
"The chances of finding people alive in these conditions, in this type of setting, well, it's challenging.
"We need to hope and we need to work and we need to believe there's someone out there alive who we can save."
Italy is vulnerable to earthquakes, sitting on two fault lines.
Some in the Italian press wondered why more had not been done to improve building standards in quake prone-areas.
"I have to say that the anger is stronger than the pain - the anger at the thought that this destruction could have been avoided," Dacia Maraini said in the Corriere Della Sera.
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