Fighting in Syria's largest city, Aleppo, rages on after two weeks of clashes, with activists reporting rebels using a tank to fire on a military airport.

The Britain-based Syrian Observatory for Human Rights also says government forces shelled an Aleppo neighborhood Thursday, killing seven civilians.

Another opposition activist group, the Local Coordination Committees, said fighting killed 15 people Thursday in Damascus and six others in Homs. The group also reported warplanes flying over Deraa.

Syrian President Bashar al-Assad has urged his security forces to “step up the fight” against the rebels trying to push him from power.

The United States condemned Mr. Assad's message Wednesday calling on his armed forces to keep fighting. The Syrian leader has not been seen since unknown rebels assassinated four members of his inner circle in a Damascus bombing last month.

U.S. State Department spokesman Patrick Ventrell called Mr. Assad's actions “cowardly” and “despicable.”