BEIRUT, Lebanon — Fire swept through the old central souk, or marketplace, of Aleppo, Syria, on Saturday, damaging a vast and well-preserved labyrinth of medieval storehouses, shops, schools and ornate courtyards as fierce clashes between security forces and insurgents vowing to carry out a “decisive battle” for the city continued.

One video shot by antigovernment activists showed a curtain of dark smoke hanging over the center of Aleppo near the old city, a Unesco World Heritage site. Another showed intense, crackling orange flames engulfing heavy wooden doors in what appeared to be one of the market’s arched stone passageways. The activists said they believed that the fire, whose origins were unclear, had destroyed a large portion of the market’s shops overnight, though the claim could not be immediately verified.

For many residents, the old city, with the souk at its center, is the soul of Aleppo, one of the world’s oldest continuously inhabited cities and Syria’s largest. Aleppo has been staggering for months under a bloody battle that has reduced some residential areas to rubble, and with no deaths immediately reported from the blaze, the damage to the souk pales compared with the recent human toll.

Yet serious damage to an area that Syrians widely consider one of their greatest treasures is likely to stir anger at both sides — each of which blames the other for the destruction in the city — in a conflict that seems mired in stalemate. It could also make the rebels’ latest push in Aleppo backfire politically: Some opponents of President Bashar al-Assad were already incensed on Saturday at insurgents they said had operated conspicuously near the old city.