(The identity of the reporter has been withheld for security reasons)
* Assad set on controlling Aleppo to protect Alawite coast
* Syria fragmenting into Alawite, Islamist and Kurdish areas
* Civilians escaping barrel bombs face violence, alienation
ALEPPO, Syria, March 25 (Reuters) - Aleppo is central to President Bashar al-Assad's goal of rescuing a viable state from the ruins of Syria; hence the grim message from his forces to its residents, that one equals five.
"We told them every shell (they fire) equals five barrel bombs," said Amar, a local policeman in the city, who argued that any civilians hit by the highly destructive improvised weapons deserved it for tolerating "terrorists".
"They didn't believe us and they continued launching shells, so the army responded with a pounding of barrel bombs."
Almost two years after rebels grabbed half of Syria's biggest city, they are on the defensive, with government forces advancing on three sides.
If Assad can retake Aleppo, he would be back in control of Syria's three largest cities, a bulwark for the Mediterranean provinces of Latakia and Tartous which form the heartland of his minority Alawite sect.
With the rest of the country split between autonomous Kurdish areas in the northeast and a range of Sunni Islamist rebel groups in the east, Syria's fragmentation could become irreversible.
The battle for Aleppo ebbs and flows - the rebels regained some high ground at the weekend - but Assad's forces struck back on Monday, dropping barrel bombs from helicopters on several rebel-held districts in the east.
The military ramped up its offensive in December, pummeling civilian areas with scores of barrel bombs, made from oil drums packed with explosives and shrapnel that cause massive and indiscriminate destruction.
In six weeks, they killed more than 700 people, mostly civilians, and forced tens of thousands more from their homes.
Khadija and her six children fled their home in the eastern district of Al-Sukkari when it was struck by a barrel bomb in late January.
They passed through the "death crossing" - a 100-metre stretch of sniper territory between Aleppo's eastern and western halves - hoping for a better life on the other side.
"When we reached the government side, the soldiers viciously beat us with a stick," she said.
Denied a residency permit to live in government-held territory, they sleep wherever they find shelter, moving every few days to evade security forces.
New York-based Human Rights Watch said on Monday that satellite images showed 340 sites hit in opposition-controlled Aleppo between early November and late February. The damage appeared "strongly consistent with the detonation of highly explosive unguided bombs."
Western powers have condemned the use of barrel bombs as a war crime, but they continue to fall nearly every day in Aleppo and other parts of Syria where more than 140,000 people have died in three years of war.
The bombardment has uprooted thousands of people, some of whom fled to neighbouring Turkey while others, like Khadija, moved to government-controlled areas of Aleppo, where they have been forced to camp on the street and in parks and schools.
Abeer, an Aleppo-based researcher with the Jesuit Refugee Service, said Assad's forces were even bombing government-held districts once controlled by rebels as part of what she called a government "policy of collective punishment".
"They continue to strike neighbourhoods with barrel bombs to punish their residents for embracing the opposition fighters when they entered," she told Reuters.
Some of those forced to flee live on the streets, often with only a flimsy tarpaulin for shelter. Others have sought cover in school buildings, packed into classrooms by the dozen even as pupils attend lessons, stirring social tension in a city once reknown for its religious and political diversity.
"Aleppo is enduring a dreadful type of social fragmentation because of the hatred between its residents, and the increased number of displaced people has deepened this fragmentation," Abeer said.
Abdel Jabar and his family escaped a barrel bomb attack in January, but have since lived as outcasts in a public garden on the other side of the city.
Security forces forbade them from living with relatives in a government-held district, he said. "The authorities impose residency restrictions on us as if we are strangers in our own country." (Writing by Stephen Kalin; editing by Dominic Evans and Philippa Fletcher)