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They fall randomly, often during rush hour, smashing into schools, businesses, churches and homes in the Syrian capital and leaving a trail of death and terror in their wake.

As President Bashar Assad's forces press ahead with a crushing offensive, rebels are increasingly hitting back by sending volleys of mortar shells crashing into central Damascus, hoping eventually to loosen the leader's firm hold on the capital.

Dozens of shells have struck the historic city center and surrounding areas in the past two weeks, launched from rebel-held neighborhoods on the outskirts. Mortar shells, many homemade, are known to be imprecise so it's not possible to determine whether the rebels fire them randomly or at specific targets.

Much of the shelling in recent weeks has centered on Christian-majority areas in the Old City, where many residents now barricade themselves at home and schools are half empty. Businessmen say sales have plunged, because people no longer dare to go out.

"Shells have become part of our everyday life," said Jean Nahhas, an 18-year-old business administration student who lives in the predominantly Christian area of Qassaa, which has been particularly hard hit by the shelling.

Nahhas, whose uncle was killed in shelling a month ago, said he and other Syrians have established a routine, going home early and calling to check on family and friends after each explosion.

Assad retains strong support in the capital, particularly among minority sects including Christians, Alawites, Druse and Shiites, making them likely targets. Foreign embassies and schools also are frequently hit. Rebels are overwhelmingly from the country's Sunni majority sect, and Christians are convinced that Islamic extremists among the fighters are deliberately targeting their neighborhoods.

"They fire randomly to force Christians to leave," Damascus-based Greek Orthodox Patriarchal Assistant Bishop Luca al-Khoury said in a telephone interview.

On Thursday, a mortar shell landed inside the Russian Embassy in Damascus, killing one Syrian and wounding nine, including guards, the Russian Foreign Ministry said. The embassy building sustained minor damage.

More than 100 people have been killed in mortar attacks on Damascus since this summer, according to reports by the Syrian state news agency SANA and the British-based activist group, the Syrian Observatory for Human Rights. That has punctured the sense of normalcy that the government has tried hard to cultivate. For the first two years after the uprising against Assad began in March 2011, the ancient city with its historic markets, mosques and churches was largely insulated from the bloodshed and destruction that plagued other areas.

Syria's deputy foreign minister, Faisal Mekdad, pointed to the recent "dangerous escalation" in mortar strikes during a meeting Thursday with foreign ambassadors and criticized the failure of Western countries to condemn them.

The death toll and damage is relatively minor compared to other, opposition held urban centers, which have been battered — in some cases entire blocks have been leveled — by Syrian army warplanes, heavy artillery and street fighting.

The rebels launched an offensive on Damascus in July 2012, but were swept out in a punishing counteroffensive. Since then, government warplanes have pounded opposition strongholds on the outskirts, and rebels have managed only small incursions on the city's southern and eastern sides. The government has made significant advances against rebels in recent weeks — and the rebels have stepped up the mortar fire.

Nobody claims responsibility for the attacks that target residential areas or kill civilians.

Islam Alloush, a spokesman for the Islamic Front, an umbrella rebel group, insisted only military targets are legitimate.

Alloush, however, said informal statistics show some 8,810 rebel units are operating in the Damascus area, and "unfortunately there are some who don't care about killing people, they do it out of ignorance or spite." He said the group was trying to set up a supreme military court to control violations.

There is no telling where the mortar shells — easily launched weapons that spray large pieces of shrapnel upon explosion — will fall. Almost every Damascene has a story about a mortar shell affecting their lives.

"My wife was hanging clothes when the shell crashed through the ceiling of our apartment," a 57-year-old businessman named Marwan said, adding the blast destroyed furniture, air conditioners "The furniture has been destroyed, the ACs, the walls are cracked," lamented the man who has moved with his wife and son with his sister until their home is repaired. Like many others in Syria, he spoke on condition that only his first name is used, fearing reprisals.

In one of the deadliest attacks, mortar shells struck a primary Christian school in Qassaa in Damascus on Nov. 12, killing five children and their bus driver and wounding 27. Another rocket smashed near a school bus in Bab Touma the same day, injuring five students.

Jaramana, a town just southeast of the capital with a mix of Druse and Christians, also is a frequent target of mortar shells and car bombs.

Mai, a 57-year-old teacher whose teenage niece was killed by shrapnel a month ago, said the absence rate among her school's 40 students has risen to 25 percent as many parents are too afraid to let their children leave the house.

"The students are in a permanent panic... their comprehension ability has notably gone down," she said.

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Karam reported from Beirut.