Syrian President Bashar al-Assad has 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 says 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 Mr. Assad did not deliver the Armed Forces Day message in person or on state television.

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. Witnesses said rebels executed some government prisoners, but the reports could not be independently verified.

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.

Elsewhere, the Britain-based Syrian Observatory for Human Rights said at least one person was killed Wednesday in fighting between rebels and security forces in a predominantly Christian neighborhood of Damascus. Fighting has been rare in Christian areas of the capital during the 17-month uprising against Mr. Assad's rule.

The Observatory says more than 20,000 people have been killed since the revolt began.