A strong tremor has shaken quake-hit areas of central Italy and sent rescuers and residents fleeing from damaged buildings in L'Aquila.

Chunks of concrete dropped from the buildings there and the shock was felt as far away as Rome.

Firefighters were attempting Tuesday to reach four students located trapped in a dormitory that crumbled in Italy's devastating earthquake. It was not known if they were alive or dead.

Chief firefighter Sergio Basti said rescue crews were evacuating the area around the dorm because they plan to begin "surgically" removing big chunks of the building to reach the four students.

Asked if the students were alive, Basti said only that in his experience he had found people alive after 14 days.

He said Tuesday that rescue crews needed to "lighten" the building to reach the four since they are in a hard-to-reach spot. Several dozen rescue crews are at the scene of the collapsed building.

Relatives of the missing watched agonized as rescuers dug desperately by hand for survivors, jarred by a strong aftershock that drove home the continuing danger.

The death toll from Italy's worst earthquake in three decades jumped to 235 as bodies were recovered and identified.

Lilly Centofanti waited with her mother on the lawn in front of a partially collapsed university dormitory for word of her 19-year-old younger brother, Davide, who lived on the third floor.

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Centofanti and her mother comforted each other as relatives called the younger woman's cell phone for updates.

"There's no information," she kept saying.

"We're waiting," she told a reporter."We only know the shocks go on."

Premier Silvio Berlusconi surveyed the devastated region by helicopter and said the rescue efforts would continue for two more days — after which any of the trapped would have little chance of survival. Fifteeen people were still missing, he said.

"The rescue efforts will continue for another 48 hours from today until it is certain that there is no one else alive," Berlusconi told reporters.

Berlusconi said that at least 100 of the roughly 1,000 injured people were in serious condition. As many as four students could still be inside the dormitory in L'Aquila — a central Italian city of Romanesque, Gothic, Baroque and Renaissance architectural treasures, Berlusconi said.

A series of aftershocks have hit of L'Aquila and 26 surrounding towns and cities in the snowcapped Apennine mountains since the quake early Monday, which also left tens of thousands homeless. Tuesday's aftershock appeared strongest around L'Aquila, a city of some 70,000 people.

Two buildings in Pettino, a suburb of L'Aquila, collapsed following the aftershock, the news agency ANSA reported, citing fire officials. No one was believed to be inside either building.

The ground shook in the nearly leveled town of Onna, about six miles away, but caused no panic.

Rescuers were still trying to reach more isolated hamlets on Tuesday.

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Officials said some 10,000 to 15,000 buildings were either damaged or destroyed, and at least 50,000 people were left homeless. In Onna, 38 people out of some 300 inhabitants were dead, rescue officials said.

While the elderly, children and pregnant women were given priority at tent camps in the area, others were sleeping in cars or making their own arrangements to stay with relatives or in second homes out of the quake zone.

Six months pregnant, Sandra Padil spent the night in a tent without any covers in the chill mountain air as the temperatures dipped to 43 degrees.

"We are calmer out in the open," said Padil, a 32-year-old Peruvian who has been living in L'Aquila since 1996. "We didn't have blankets and it was cold, but at least this morning they gave us breakfast. Let's hope this ends quickly."

Some elderly people appeared to be disoriented as they walked among the tents, and people tending them complained about the lack of blankets.

Mounting piles of rubble contained evidence of shattered lives: torn clothing, ripped stuffed animals and broken furniture.

The U.S. Geological Survey said the main quake — which struck just after 3:30 a.m. Monday as most people slept — was magnitude 6.3 on the so-called "moment scale," but Italy's National Institute of Geophysics, using the Richter scale, put it at 5.8.

Rescue workers arrived from throughout Italy, from as far away as Venice and Genoa. Part of L'Aquila's main hospital was evacuated for fear of collapse, and few operating rooms were in use. Bloodied victims waited in hospital hallways or in the courtyard and many were being treated in the open.

Law enforcement placed cordons around the areas hardest hit by the quake to prevent looting, including the center of L'Aquila and the towns of Paganica and Onna, Capt. Ivan Centomani of Italy's financial police told Sky Italia TV from L'Aquila.

Italy's national police chief, Antonio Manganelli, said several people had been arrested for looting from abandoned houses.

The quake took a severe toll on L'Aquila's prized architectural heritage. Many Romanesque, Gothic, Baroque and Renaissance landmarks were damaged, including part of the red-and-white stone basilica of Santa Maria di Collemaggio.

The bell tower of the 16th-century San Bernardino church and the cupola of the Baroque Sant'Agostino church also fell, the Culture Ministry said. Stones tumbled down from the city's cathedral, which was rebuilt after a 1703 earthquake.

Damage to monuments was reported as far away as Rome, where cracks appeared at the thermal baths built in the 3rd century by the emperor Caracalla, Culture Ministry official Giuseppe Proietti said.

Berlusconi declared a state of emergency, freeing up millions in euros to deal with the disaster, and canceled a visit to Russia so he could deal with the crisis.

Condolences poured in from around the world, including from President Barack Obama, Pope Benedict XVI and Abdullah Gul, president of quake-prone Turkey.

It was Italy's deadliest quake since Nov. 23, 1980, when a 6.9-magnitude quake hit southern regions, leveling villages and causing some 3,000 deaths.

The last major quake to hit central Italy was a 5.4-magnitude temblor that struck the south-central Molise region on Oct. 31, 2002, killing 28 people, of which 27 were children who died when their school collapsed.