* Safe corridors set up for civilians to free
* But people trapped by artillery fire and snipers
* Retaking city would be Assad's biggest victory so far (Adds rebel comment & planes drop diapers in opposition areas)
BEIRUT/MOSCOW, July 28 (Reuters) - Pro-government forces tightened their grip around the besieged Syrian city of Aleppo on Thursday where the local governor said safe corridors had been opened for civilians to escape rebel-held areas.
A quarter of a million civilians still live in Aleppo's opposition-controlled eastern neighbourhoods, effectively under siege since the army and allied militia cut off the last road into rebel districts in early July.
State television quoted the governor of Aleppo as saying three humanitarian corridors would be established and President Bashar al-Assad offered an amnesty for rebels who surrender within three months.
His ally, Russia, said a fourth corridor would be set up in the north of the city for surrendering rebels, near the Castello road which the army recently seized.
Pictures of what appeared to be a leaflet dropped on Thursday showed a map of Aleppo, entitled "Safe exit points from Aleppo city", with four crossing points out of the rebel areas marked as humanitarian corridors.
The Syrian army said on Wednesday it had dropped thousands of leaflets over opposition-held Aleppo districts, asking residents to cooperate with the military and calling on fighters to surrender.
But two rebels and aid workers contacted in besieged Aleppo said the army fired at civilians at one of the safe corridors, in the Salah al Din district.
A doctor for a medical charity that operates in Aleppo also said the army had fired artillery at families gathering near another humanitarian corridor, in the opposition-held Bustan al Qasr neighbourhood.
Hael Asi Hilal, head of the Syrian Red Crescent in rebel-held areas, said no family so far had been able to leave to government-held areas via any corridor due to snipers firing at them.
The UK-based Syrian Observatory for Human Rights, said helicopters over rebel-held areas dropped baby diapers and meal packs that had Russian language labels.
Russian Defence Minister Sergei Shoigu said: "On behalf of the President of the Russian Federation, today, (we will) start a large-scale humanitarian operation together with the Syrian government to help civilians in Aleppo."
Aleppo, Syria's biggest city before the outbreak of the conflict five years ago, has been divided between government forces and rebels since the summer of 2012. Its recapture would mark Assad's biggest victory so far in the civil war.
The army, backed by allied militia forces and air support from Syrian and Russian jets, has taken more ground on the northern edge of the city, around the Castello road which leads out towards Turkey.
State television said the army had advanced in the Bani Zeid district, on the southern side of the Castello road. The Syrian Observatory for Human Rights said the pro-government forces had taken full control of the district.
A rebel source confirmed that the army had made advances. He said Kurdish forces from the nearby Sheikh Maqsoud district had also taken advantage of the fighting to advance into a housing complex in Bani Zeid.
"There has been a (rebel) withdrawal, but no one has surrendered," Zakaria Malahifji of the Aleppo-based rebel group Fastaqim group told Reuters.
(Reporting by Dominic Evans in Beirut and Alexander Winning in Moscow. Additional reporting by Suleiman Khalidi and Lisa Barrington in Beirut; Editing by Hugh Lawson and Robin Pomeroy)
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