Activists say thousands of troops have been sent to Syria's second city, Aleppo, as clashes were reported in the city for the sixth consecutive day.
Fighting was reported in the central al-Jamaliya neighbourhood on Wednesday, close to the local headquarters of the ruling Baath party. In Kalasseh, in the south of the city, rebels set fire to a police station, the Syrian Observatory for Human Rights said.
The 16-month revolt against President Bashar al-Assad has in recent weeks been transformed from an armed uprising in remote provinces into a battle for control of the two main cities, Aleppo and the capital, Damascus, which have been the regime's main bases of support.
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A spokesman for rebel Free Syrian Army said thousands of Syrian soldiers had been moved from the northwestern province of Idlib to fight in Aleppo.
"A large number of troops is being redeployed from Jabal al-Zawiyah to Aleppo, which is strategically more important for the regime than Idlib," Colonel Abdel Jabbar al-Oqaidi, the FSA's Military Council spokesman in the city told AFP.
Activists said people were fleeing the southern neighborhood of Sukkari on Wednesday morning.
Al Jazeera’s Rula Amin, reporting from Beirut in neighbouring Lebanon, said the battle for Aleppo is critical for the opposition and the government.
“For the opposition to win this revolution, they have to win Aleppo," she said. "They have to make sure that this large city, this commercial hub, joins in. This is why, for the government also, it’s a very decisive battle and they’re not sparing any effort or any weapon to crush the rebels."
The Observatory said more than 30 people died in violence across the country on Wednesday, a day after 158 people were reported to have been killed nationwide.
The Britain-based group also reported clashes in the district of Hajar al-Aswad in Damascus, one of the last remaining rebel bastions in the capital, 10 days after fighting broke out there.
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Regime forces used helicopter gunships and heavy machinegun fire to pound the embattled southern neighbourhood, the Observatory said.
Activists and residents also said Syrian forces fired artillery and rockets at the northern Damascus suburb of al-Tal in an attempt to seize it from rebels, causing panic and forcing hundreds of families to flee.
"Military helicopters are flying now over the town. People were awakened by the sound of explosions and are running away," Rafe Alam, an activist, said by phone from a hill overlooking al-Tal. "Electricity and telephones have been cut off."
In Aleppo, al-Oqaidi said he believed the reinforcements were being sent to Aleppo because of the intensity of clashes in the city, where several districts were "liberated" on Monday.
"There are clashes right now in Aleppo, so fierce that many of their troops are running away, while dozens of others are defecting on the spot," he said. "Their morale is very low."
He had earlier announced "the start of an operation aimed at liberating Aleppo from the blood-stained hands of Assad's gangs", referring to loyalists of Assad.
A commercial hub and home to 2.5 million people, Aleppo recently became a new front in the country's uprising, after being largely excluded from the violence for more than a year.
Elsewhere in the province of Aleppo, residents accused government troops of indiscriminately firing on the town of al-Jinah.
Amateur video obtained by Reuters news agency appeared to show the aftermath of fighting in which residents said three people were killed and another wounded when a car was hit by a mortar shell.
Opposition activists said government troops and rebels have fought fierce battles in the area.
"What is the fault of this village to be hit with these shells? Ten shells on a daily basis and this village does not have any sign of armed groups?" a resident said, saying the village had been targeted "only because we called for freedom".
Also on Wednesday, security forces continued to fight with prisoners at the central prison in Homs, after a mutiny that saw detainees take over a wing.
The Observatory said security service agents and regular troops took part in the operation, which left several "dead and wounded".
The mutiny broke out last week, and was followed by a similar revolt in Aleppo's central prison.
In the city of Homs, a rebel fighter was shot dead by a sniper in the al-Qarabis district, the Observatory said, adding that regime forces were firing an average of "three shells every 15 minutes".