Mystery surrounded the whereabouts of Syrian President Bashar al-Assad on Thursday, as battles raged in the center of Damascus a day after a bomber killed his top security chiefs.

The Syrian leader made no public appearance and no statement after a bomb in the heart of the capital killed his powerful brother-in-law, his defense minister and a top general, drawing fierce army retaliation with heavy artillery against the rebels.

On Thursday morning, residents said there was no let-up in the heaviest fighting to hit the city in a 16-month revolt against Assad’s rule, now into its fifth day.

The fighting came within sight of the presidential palace, near the security headquarters where the bomber struck a crisis meeting of defense and security chiefs.

Residents in the Midan and Kafr Souseh districts said they heard explosions and heavy gunfire as helicopters buzzed overhead.

Checkpoints around Midan and the ancient walled Old City ofDamascus had been removed, they said. It was unclear if security forces had changed tactics to prevent rebels from targeting soldiers, or if it was a temporary move in the heat of battle.

On Wednesday, an explosion which nearly no residents heard struck at the heart of Assad’s government. Following the attack, rebellious neighborhoods were plunged into fierce battles.

Assad’s brother-in-law Assef Shawkat, a top commander and one of the pillars of the Assad clan’s rule, was killed in the Wednesday blast along with Defense Minister Daoud Rajha.

Another senior general was also killed and the heads of intelligence and the Interior Ministry were wounded, deeply damaging the security apparatus of the Assad family, which has ruled the country with an iron fist for four decades.

The army shelled its own capital from the surrounding mountains as night fell on Wednesday. Government troops, having vowed retaliation for the assassination, fired machineguns into the city from helicopters.

Rebels, massed in several neighborhoods, are armed mostly with small arms and rocket-propelled grenades.

Activist videos posted on the Internet showed bloodied bodies lying in the street.

Local media, citing security sources, said Assad was still in the capital but gave no further details. Agencies.