BEIRUT – Syrian rebels penetrated checkpoints and concrete barriers Wednesday to attack the country’s military headquarters in an assault that left the facility scorched and abandoned, and punctured President Bashar Assad’s claim to have his capital under tight control.

The attack in the heart of the city was the rebels’ second major effort in Damascus in as many days, and appeared aimed at regaining momentum after the conflict had settled in recent weeks into a stalemate across many parts of the country.

Witnesses said gun battles resounded in the area near Umayyed Square for five hours after two bomb explosions early Wednesday, as security forces tried to flush out rebel fighters. Residents were ordered by loudspeaker to stay inside and shut their doors.

Unlike a bombing in mid-July that officials said killed four of Assad’s top security aides, the government said Wednesday’s attack did not cause major casualties. But it did shatter the impression of calm and control that the government has tried hard to create since heavy fighting this summer in which Assad’s forces claimed to have pushed rebels out of districts across the city.

The attack came as the United Nations General Assembly has been debating the crisis in Syria. A deeply divided U.N. has failed to agree on sanctions to punish Assad, and efforts to negotiate an end to the fighting have gone nowhere.

Wednesday’s attack occurred only about a mile from Assad’s residence, the most visible rebel strike to date in the center of the capital. Intense fighting in July and August focused on outlying districts and working-class suburbs where the rebels had established a presence.

The official press agency said four guards were killed and 14 other people were injured in the strike against the armed forces command headquarters. State media said that no military commanders had been killed or injured, and that “the terrorists as usual failed to achieve their goals.”

The rebel Free Syrian Army claimed responsibility for the two bombs and said “dozens” had been killed. A rebel group calling itself the Mustafa Brigade said the blasts were caused by a pair of car bombs, at least one of which was a suicide attack. Rebels also claimed to have planted explosives inside the military headquarters.

Fighting rages on in various areas of the country, including the northern city of Aleppo, the central city of Homs and the eastern city of Dair Elzur.

The 18-month rebellion against Assad, whose family has ruled Syria for more than 40 years, is estimated to have left more than 20,000 dead.