The explosions killed six prisoners, said the British-based Syrian Observatory for Human Rights, which communicates with a network of activists on the ground. The explosives hit on Friday night, the Observatory said. It was not clear who fired the shells.
The Observatory reported about 70 soldiers and fighters were killed on Sunday, as well as 40 civilians, in fighting across Syria. The U.N. estimates some 93,000 people have been killed in the civil war.
With government forces stepping up offensives, the Syrian Muslim Brotherhood called on the U.S. and Europe to send arms.
"Providing the Free Syrian Army and the revolutionary rebels with appropriate arms is more urgent now than at any time in the past," the movement wrote on social media sites. "We feel cheated and disappointed because the U.S. and Europe have backed out from arming the FSA," it said.
Last month the U.S. decided in principle to provide some weapons to rebel forces, though Western countries are concerned they might land in the hands of extremist Sunni Muslims fighting with the rebels.
The forces include an al-Qaida-linked group which has been fighting for weeks to seize control of the prison in Aleppo, besieging it. The Observatory estimated some 120 prisoners have died in the jail since April from fighting, illness and executions.
Syria's state run news agency SANA said "a number" of rebels were killed in the shelling but did not give an exact number.
Aleppo, Syria's largest city, is near the border with Turkey. Many of its ancient monuments and its marketplace, once a magnet for tourists, have been destroyed in fighting.
Rebels and government forces also clashed near the Shiite towns of Nubul and Zahra in Aleppo province, the Observatory and pro-rebel activists reported. The towns have been besieged since at least May by hard-line Sunni rebels seeking to dislodge their enemies.
The Observatory said fighting killed three regime troops, including one foreigner, code for a fighter from the Lebanese Shiite Hezbollah.
Rebels claim that Assad's forces and Hezbollah fighters are in the two towns. A hard-line Sunni brigade warned last week it would punish Shiites for harboring the forces, suggesting the towns' populations of some 40,000 Shiites could be targeted.
The fighting underscores the growing sectarian nature of the two-year uprising against Assad's regime. It began as peaceful protests but turned into an armed rebellion after a brutal government crackdown. It has since taken on regional dimensions, with Hezbollah fighters joining Assad's forces. Foreign Sunni fighters have joined predominantly Sunni Syrian rebels who are formed in bands ranging from secular to hard-line Islamists.
At home, Assad draws support largely from Syria's minorities, including fellow Alawites—followers of an offshoot of Shiite Islam—as well as Christians, Shiites and Sunnis who fear the hard-line rebels.
In recent weeks, Assad's forces, bolstered by Hezbollah fighters, have pushed back to seize rebel-held areas in several parts of Syria.
In the central Syrian city of Homs, Assad's forces fired mortar shells from a stronghold of buildings on the edge of the rebel-held area of Khaldiyeh, trying to flush out fighters, said two activists.
Explosions could be heard as they spoke via Skype.
The shells were exploding in the densely-built area surrounding the 13th-century mosque of Khalid Ibn al-Walid, famous for its nine domes and two minarets, said a Homs-based activist identified as Nedal. He said parts of the wall surrounding the historic complex were blown away. Other parts were damaged in previous rounds of fighting.
Khaldiyeh-based activist Abu Bilal said fighters were low on weapons. He said the international community, despite promises to arm rebels, had left them hanging in Homs.
"They have sold Homs to the enemy," he complained.
The U.N. warns the some 4,500 residents in besieged, rebel-held areas of Homs face a humanitarian catastrophe. On Friday, the divided U.N. Security Council failed to approve a statement calling on the Syrian government to allow immediate access to trapped civilians there. Russia, Syria's closest ally, demanded that the statement should also call for immediate access to the towns of Nubul and Zahra.
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