From our partners at the BBC:
A magnitude 6.2 earthquake has struck central Italy, leaving at least 21 people dead and many others trapped under rubble, reports say.
"Half the town is gone," said the mayor of Amatrice, one of the worst areas affected.
A family of four were feared dead in the rubble of their house in the nearby town of Accumoli.
The quake hit at 03:36 (01:36 GMT), 100km (65 miles) north-east of Rome, at a shallow depth of 10km.
Some buildings in the capital shook for 20 seconds.
Officials warned the number of fatalities was likely to rise. The head of the civil protection department compared the earthquake's intensity to Aquila in April 2009 in which 309 people died.
Some of the worst damage was in the town of Amatrice, where at least five died and rescue efforts were under way to find survivors.
"The roads in and out of town are cut off. Half the town is gone. There are people under the rubble. There's been a landslide and a bridge might collapse," said mayor Sergio Pirozzi.
"There are tens of victims, so many under the rubble. We're preparing a place for the bodies," he said.
The main street through the town has been devastated and emergency workers are trying to reach six people in a collapsed building.
In Accumoli, a short distance to the north, the mayor said six people had died.
"One person was pulled out of the rubble during the night," said Stefano Petrucci.
"Then there is a family of four under a collapsed house and sadly there are two small children among them."
Meanwhile reports said 10 people had died in the village of Pescara del Tronto.
Seismologist Andrea Tertulliani said there were sure to be further, numerous shocks that would probably diminish in intensity.
"But it can't be ruled out that there could be another shock on the same scale as the main one," he said.
Italy's Civil Protection agency described the earthquake as "severe".
"It was so strong. It seemed the bed was walking across the room by itself with us on it," Lina Mercantini of Ceselli, Umbria, told Reuters.
Rescue teams are being sent to the worst-hit areas, the prime minister's office said.
The quake was initially reported as being magnitude 6.4. It was followed by several powerful aftershocks, La Repubblica newspaper reported.
The deputy editor of the British newspaper, The Times, who was in the area at the time, told the BBC that the quake lasted about 20 seconds followed by an aftershock about 20 minutes later which was easily as strong.
"It was pitch dark, very cold. Nobody in our group had a clue what to do in an earthquake," Emma Tucker said.
The USGS predicted the damage could be significant, based on data from previous quakes.
It said the quake struck near the Umbrian city of Norcia, whose picturesque historic centre is a popular tourist site.
However Norcia Mayor Nicola Alemanno said no deaths have been reported there.
"The anti-seismic structures of the town have held," he said. "There is damage to the historic heritage and buildings, but we do not have any serious injuries."
In 2009, a 6.3-magnitude earthquake in the Aquila region, which was also felt in the Italian capital, left more than 300 dead.