(CNN) A powerful 6.6-magnitude earthquake rocked central Italy on Sunday morning, injuring at least 20 people, in the strongest tremor to hit the country in more than three decades.

The earthquake follows tremors last week and comes on the heels of a devastating quake in August , which killed nearly 300 people and flattened entire villages.

Firefighters from Vatican City check out St. Paul's Basilica in Rome on October 30 following the earthquake. Several buildings in the Italian capital have suffered minor damage from the series of quakes.

Firefighters from Vatican City check out St. Paul's Basilica in Rome on October 30 following the earthquake. Several buildings in the Italian capital have suffered minor damage from the series of quakes.

An aerial view shows the destruction in the hilltop town of Amatrice following the earthquake on October 30.

An aerial view shows the destruction in the hilltop town of Amatrice following the earthquake on October 30.

The facade of the Basilica of San Benedetto in Norcia remains standing on October 30, but much of its core has collapsed.

The facade of the Basilica of San Benedetto in Norcia remains standing on October 30, but much of its core has collapsed.

A group of nuns leave the center of Norcia on October 30 after being rescued earlier. Residents ran onto the streets in a panic when the quake struck in the morning.

A group of nuns leave the center of Norcia on October 30 after being rescued earlier. Residents ran onto the streets in a panic when the quake struck in the morning.

A man rides a bicycle past cracks in a road in Norcia on October 30. The quake struck 6 kilometers (3.7 miles) north of Norcia, the US Geological Survey reported.

A man rides a bicycle past cracks in a road in Norcia on October 30. The quake struck 6 kilometers (3.7 miles) north of Norcia, the US Geological Survey reported.

The tower of the Church of the Madonna of the Angels remains standing amid rubble near Norcia on October 30.

The tower of the Church of the Madonna of the Angels remains standing amid rubble near Norcia on October 30.

A firefighter and a soldier survey rubble in Amatrice on October 30. The latest quake on Sunday was the strongest to hit the country in more than three decades. Two other quakes struck the same region within the past week. Amatrice's town center also was badly damaged in a deadly temblor in August.

A firefighter and a soldier survey rubble in Amatrice on October 30. The latest quake on Sunday was the strongest to hit the country in more than three decades. Two other quakes struck the same region within the past week. Amatrice's town center also was badly damaged in a deadly temblor in August.

Residents from the village of Caldarola prepare to spend the night in an emergency camp set up in a warehouse on October 30.

Residents from the village of Caldarola prepare to spend the night in an emergency camp set up in a warehouse on October 30.

A general view of Arquata del Tronto shows the extent of damage to the town in the region of Perugia on Sunday, October 30.

A general view of Arquata del Tronto shows the extent of damage to the town in the region of Perugia on Sunday, October 30.

A statue of St. Benedict remains standing in front of the damaged bell tower in the town of Norcia on October 31.

A statue of St. Benedict remains standing in front of the damaged bell tower in the town of Norcia on October 31.

Italian authorities register people for temporary accommodations in the Adriatic coastal town of Porto Sant'Elpidio on October 31, after more than 15,000 were displaced by the latest earthquakes to hit central Italy.

Italian authorities register people for temporary accommodations in the Adriatic coastal town of Porto Sant'Elpidio on October 31, after more than 15,000 were displaced by the latest earthquakes to hit central Italy.

A graveyard in Campi, central Italy, lies in ruins on Monday, October 31, after another powerful earthquake struck the region on Sunday. No deaths have been reported from the 6.6-magnitude quake as many towns in the affected area were evacuated following a devastating earthquake in August, which killed almost 300 people.

A graveyard in Campi, central Italy, lies in ruins on Monday, October 31, after another powerful earthquake struck the region on Sunday. No deaths have been reported from the 6.6-magnitude quake as many towns in the affected area were evacuated following a devastating earthquake in August, which killed almost 300 people.

Latest developments

• The quake struck 6 kilometers (3.7 miles) north of Norcia, the US Geological Survey reported.

• Fire and rescue services said six people had been pulled from rubble in Norcia.

• There were no immediate reports of deaths -- many residents had not returned since a devastating quake in August.

• There have been about 200 aftershocks since Sunday's quake in the border area between the Marche and Umbria regions, according to National Institute for Geophysics and Vulcanology.

• Some villages are cut off, so the impact there has not been assessed, said Fabrizio Curcio, the civil protection chief.

• Some 15,000 people are without electricity, according to Curcio.

• Much of the Basilica of San Benedetto in Norcia collapsed.

Sunday's quake struck at a shallow depth, making it more likely to cause damage.

Prime minister: 'We will rebuild everything'

Residents ran onto the streets in a panic as the quake struck at 7:40 a.m. local time (2:40 a.m. ET) Sunday, and rescuers were seen soon after, helping evacuate a group of nuns from a church. They worked through aftershocks that hit every 20 minutes or so.

Nuns leave the center of Norcia after being helped out of a church Sunday.

Authorities are hopeful there may be no deaths since many residents in the region had already been evacuated to emergency camps and hotel rooms paid for by the government after the August quake and last week's temblors, and schools had shut down in anticipation of powerful aftershocks.

Italian Prime Minister Matteo Renzi in Rome thanked rescuers and vowed to rebuild damaged villages.

"Italy has many faults, but these situations bring out the best of us," he said, adding the country would rebuild houses, churches and shops.

"We will rebuild everything. We have the resources to do it."

Italian civil protection staff pass a collapsed wall Sunday in Norcia after the quake jolted central Italy.

The main road into the worst-affected area has been blocked at various points, said Curcio, the civil protection chief, and helicopters were airlifting the injured to the hospital in Foligno, around 30 kilometers to the northwest. Six aircraft were monitoring the affected regions, he added.

Initial images from Sunday's quake show devastation to some historical buildings. Many of these buildings had not been reinforced since two powerful quakes struck Wednesday.

The town center of Amatrice took another battering Sunday after most of its buildings were seriously damaged in the August quake -- a lone bell tower was seen poking up among the rubble.

The much-visited Basilica of San Benedetto in Norcia was leveled at its core, only its facade still somewhat intact. Central Italy is a deeply Catholic region.

The Basilica of San Benedetto's facade remains standing in Norcia, but its core crumbled Sunday.

Residents sleeping in cars

Father Benedetto from the monastery's Monks of Norcia told the state-run ANSA news agency: "We monks are all fine but our hearts go out to those affected, and the monks of the monastery are trying to figure out if anyone is in need of their last rites."

"We rely, as always, on your prayers and your support," he said.

He said people were still trapped in Norcia's main square, worried about more buildings crumbling should they try to flee by road.

Italian journalist Sabrina Fantauzzi tweeted a photo of a road in Norcia badly damaged in the quake.

Many others, however, are refusing to leave, some sleeping in their cars that they believe offer better protection than the roofs over their heads.

"These people don't want to leave their area because they are really linked in. There's a long tradition there, and they have some jobs, farmers and so on, so they decide to stay there," Tommaso Della Longa of the Italian Red Cross told CNN.

"People need to understand they need to move from there, because the situation in this area is between mountains and valleys -- it's cold even now, and snow is coming in a few weeks," he said, adding it was difficult to set up support structures there.

Landslides caused the partial disruption of the course of the Nera River, and the river flooded a road to the village of Visso, already full of debris. Visso was one of the towns affected by a large earthquake Wednesday.

Technical teams inspected dams, officials said, but have not found any damage so far.

'The whole city shook'

The quake was felt as far north as the Alps, Curcio said, and as far south as Rome, some 90 kilometers (56 miles) away. The metro in Rome has been shut down as authorities continue to survey the area, municipal officials said.

Schools in Rome will be closed Monday so buildings can be surveyed for any structural damage, according to the Rome municipality website. Several buildings have suffered minor damage from the series of earthquakes.

Sunday's quake has damaged a building in Norcia, where many people are afraid of leaving.

Morgan Kasmarik, an Australian journalist on vacation in the Italian capital with a friend, said "the whole city shook" when the quake struck.

"I was in bed, enjoying a Sunday lie-in, when I felt the room start to shake. Within seconds, the ground started to roll, like I was on a boat in choppy water," she said.

"We were both quite shaken as it was the first time either of us had ever experienced an earthquake. I'm pretty keen not to repeat the experience. (But) it didn't stop us, or the many other tourists from spilling into the streets to enjoy the beautiful day."

In central Italy, the people are accustomed to seismic events in their region, but not so many in such a short space of time.

CNN International meteorologist Derek Van Dam that Sunday's quake was Italy's strongest in 36 years and that Wednesday's temblors were "considered foreshocks" ahead of Sunday's "main earthquake."

Some 13 million people would have felt weak movement in the earth, Van Dam said, while those nearer the epicenter would have experienced strong jolts.

Sunday's earthquake struck at a depth of 10 kilometers (6.2 miles), according to the US Geological Survey, making the epicenter relatively shallow. Generally, the more shallow the epicenter, the more a quake is felt at the earth's surface, and the more damage it is likely to cause.

Photos: Central Italy shaken by 2 earthquakes A woman walks among debris at the Church of St. Antony, dating from the 14th century, in the town of Visso on Thursday, October 27, after a pair of earthquakes rocked central Italy. A magnitude 5.5 quake struck Wednesday, October 26, followed hours later by a magnitude 6.1 temblor. No deaths were reported, but historic buildings were damaged in the region where a powerful quake killed nearly 300 people in August. Hide Caption 1 of 12 Photos: Central Italy shaken by 2 earthquakes Visso was among the more heavily damaged towns, including this post office on October 27. Hide Caption 2 of 12 Photos: Central Italy shaken by 2 earthquakes Visso residents carry some of their belongings in the aftermath on October 27. Hide Caption 3 of 12 Photos: Central Italy shaken by 2 earthquakes A house lies in ruins in Visso on October 27. Hide Caption 4 of 12 Photos: Central Italy shaken by 2 earthquakes Firefighters inspect damage in Visso on October 27. Hide Caption 5 of 12 Photos: Central Italy shaken by 2 earthquakes People wander around Visso on October 27 after the quake destroyed part of their neighborhood. Hide Caption 6 of 12 Photos: Central Italy shaken by 2 earthquakes A child sleeps in a secured area after earthquakes rocked Visso on October 26. Hide Caption 7 of 12 Photos: Central Italy shaken by 2 earthquakes The Church of San Sebastiano stands amid damaged houses in Castelsantangelo sul Nera on October 26. Hide Caption 8 of 12 Photos: Central Italy shaken by 2 earthquakes Rescuers and residents inspect earthquake damage in Visso. Hide Caption 9 of 12 Photos: Central Italy shaken by 2 earthquakes The epicenter of the first earthquake was about nine kilometers (five miles) south-southwest of Visso. Hide Caption 10 of 12 Photos: Central Italy shaken by 2 earthquakes A Halloween dummy rests on a wall in front of earthquake damage in Visso. Hide Caption 11 of 12 Photos: Central Italy shaken by 2 earthquakes Residents prepare to spend the night in makeshift shelters in Visso. Hide Caption 12 of 12

Rescuers have expressed fear about possible mudslides and remained wary about the risk of bringing heavy equipment up narrow roads linking the towns, villages and hamlets in this hilly region of the country.

The two earthquakes on Wednesday -- which had magnitudes of 5.5 and 6.1 -- did not result in any reported deaths. But the destruction of nearby historic structures was widely seen as a loss to cultural heritage.