The United States has condemned remarks by Syrian President Bashar al-Assad urging his security forces to “step up the fight” against rebels trying to push him from power.

U.S. State Department spokesman Patrick Ventrell said Wednesday that Mr. Assad calling on his armed forces to keep fighting while his own whereabouts remain unknown is “cowardly” and “despicable.”

The Syrian leader has not been seen since unknown rebels assassinated four members of his inner circle in a Damascus bombing last month. On Wednesday, he failed to appear to mark his country's Armed Forces Day, instead issuing a written statement.

Fighting raged in southern parts of Syria's largest city, Aleppo, for a 13th day. It was not clear if either the government or the rebels had gained ground. Videos showed rebel fighters capturing at least two police stations from government forces on Tuesday.

Meanwhile, Syrian rebels have executed four men identified by activists as members of a pro-government tribe in Aleppo.

The Britain-based Syrian Observatory for Human Rights says a video posted on the Internet shows the rebels executing the captured government loyalists on Tuesday.

Rights groups have accused rebels of carrying out multiple abuses during Syria's 17-month uprising, while reserving the strongest criticism for security forces and pro-Assad militiamen blamed for the worst atrocities of the conflict.