BEIRUT, Lebanon — Syrian antigovernment activists said on Tuesday that an army rocket had leveled several buildings in a rebel-held neighborhood of Aleppo, killing at least 19 people and possibly leaving dozens more buried under rubble. The attack appeared to have caused one of the worst civilian tolls in the embattled city since its university was struck in a multiple bombing a month ago.

Activists also reported that up to seven mortar rounds had been fired by fighters of the Free Syrian Army toward President Bashar al-Assad’s Tishreen Palace in Damascus. There were no immediate reports of casualties, and it was not known whether Mr. Assad was there at the time. The palace, surrounded by a park, is in a wealthy area that has largely been insulated from the insurgency. It is less than a mile from the main presidential palace, which is on a plateau overlooking the city.

Suzan Ahmad, an activist in Damascus, said at least one of the mortar rounds had made a direct hit.

Syria’s official news agency, SANA, said two mortar rounds had landed near two hospitals and the palace’s southern wall, resulting in “material damage only.” It did not provide further details. SANA said the rounds had been fired by terrorists, which is what the government calls armed insurgents.

The Syrian Observatory for Human Rights, an anti-Assad group based in Britain that has a network of sources in Syria, described the Aleppo rocket as a “surface-to-surface missile” that slammed into the Jabal Badro neighborhood late Monday and said at least six children and three women were among the victims. A witness in Aleppo was quoted by Reuters as saying that the attack had felled three buildings, and that survivors were digging up bodies.