BEIRUT – Riding a wave of momentum, Syrian rebels made a run Saturday on Aleppo in some of the fiercest fighting seen in the country's largest city, which has been a key bastion of support for President Bashar Assad over the course of the 17-month-old uprising.

The rebels also took over a third border crossing – and the second one along Syria's northeastern frontier with Iraq – another sign the regime's tight grip on the country is wobbling.

The fighting in Aleppo comes on the heels of intense clashes in the capital, Damascus, as rebel forces target the pillars of regime power in their attempts to usher in what they hope will be the end of Assad's rule.