Syrian rebels have executed four men identified by activists as members of a pro-government tribe in the northern city of 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. In the footage, fighters shouting “God is Great” shove the detainees against the outside of a building before opening fire with rifles. The bodies of the men can be seen lying on the ground.

The Observatory named one of the executed men as Zeno al-Barri, the head of a militia loyal to Syrian President Bashar al-Assad. 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.

Mr. Assad's whereabouts have been unknown since rebels assassinated four members of his inner circle in a Damascus bombing last month. On Wednesday, he failed to appear in public to mark his country's Armed Forces Day, instead issuing a written statement urging security forces to “step up the fight” against rebels trying to oust him.

In the statement, Mr. Assad said his troops are engaged in what he calls a “heroic” battle whose outcome will determine the “fate of the nation.” It was the first time since succeeding his father as president 12 years ago that he did not deliver the Armed Forces Day message in person or on state television.

U.S. State Department spokesman Patrick Ventrell condemned Mr. Assad's remarks.

“We think it is cowardly, quite frankly, to have a man who's hiding out of sight be exhorting his armed forces to continue to slaughter the civilians of his own country. We think it's really despicable to be exhorting his armed forces to continue this slaughter and this bloodshed.”

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.

The U.N. observer team in Damascus reported government forces attacked rebel-held areas in Aleppo with helicopters and fighter planes. Spokesperson Sausan Ghosheh also said rebels have gained possession of heavy weapons and civilians are caught between both sides.