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The fiercest clashes yet have been reported in Aleppo, Syria's commercial capital.
Activists in Syria say there has been heavy fighting between security forces and rebels in Aleppo.
Today's clashes in the Salaheddine district are some of the fiercest so far in Syria's biggest city, local protesters told Al Jazeera.
According to the Syrian Observatory for Human Rights, based in the UK, fighting had spread to at least two other areas, Sakhour and Haydariya.
One resident of the densely populated Sakhour neighborhood described hearing continuous shelling since Friday.
"For the first time we feel Aleppo has turned into a battle zone," she told Reuters.
According to the Associated Press, the northern city has remained relatively free of violence until now. Aleppo is a large commercial hub and home to members of the elite, merchant classes and minority groups that make up President Bashar al-Assad's key support base.
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However, the AP said, in recent months students at the city's university have led growing anti-government demonstrations.
The rebels have also been increasing in strength. One activist told the AP that dozens of Free Syrian Army fighters had entered Aleppo from the surrounding countryside, where they have been gaining more and more ground.
Thousands are reported to have fled the city already. Videos posted by activists showed people leaving in cars and minibuses, the Guardian reported.
Meanwhile rebels have seized control of yet more border crossings into Iraq and Turkey. More than 100,000 Syrians are estimated to have left the country since the uprising began.
United Nations Secretary General Ban Ki-moon warned today that the violence was "destroying the country," according to CNN. Ban said Assad's government had "manifestly failed to protect civilians."
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