BEIRUT - Fires that gutted a vast medieval market have broken out in other areas of the Old City of Aleppo, a world heritage site, as rebels and government forces fight for the ancient heart of Syria's biggest city, opposition activists said on Monday.The rebels last week announced a fresh attempt to seize the city, home of a large merchant class that had mostly remained loyal to President Bashar Assad, and appeared to have focused on the warren of alleyways that make up the walled Old City.The rebels are sensitive to suggestions that they might have brought the conflict to one of Syria's greatest historic and commercial assets. Aleppo was once the last stop before Europe for traders plying the ancient Silk Route from Asia."It's urban warfare. I cannot blame any side specifically for the fires," Ameer said over Skype.UNESCO Director-General Irina Bokova said that, as a signatory to the 1954 Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict, Syria was obliged to safeguard its heritage from the ravages of war.Millenary history"The human suffering caused by this situation is already extreme," she said in a statement. "That the fighting is now destroying cultural heritage that bears witness to the country's millenary history - valued and admired the world over - makes it even more tragic."A visitor to the Old City, who asked not to be named, said the fires, which started on Saturday, were a side effect of the fighting in the covered market, famous for its silks and fabrics."An electrical fire started during clashes and spread quickly," he said, adding that several rebel groups, including those from the most prominent Tawheed Brigade, were involved in the rebel advance, which has had only marginal success in the wider city of 2.5 million people.The pattern of fighting across Syria is one of repeated destruction. When rebel fighters, lightly armed and low on ammunition, make progress, government forces respond with artillery fire until rebels and civilians are flushed out and the army can move in.But rebels are often able to sneak back, and the cycle restarts, suggesting a bleak prospect for Aleppo's Old City, where fires on Sunday reached the perimeter of the Great Mosque, parts of which date back over a thousand years.UNESCO believes that five of Syria's six world heritage sites have already been damaged. The other sites include the ancient desert city of Palmyra, the Crac des Chevaliers crusader fortress and parts of old Damascus.Some 30,000 people have now been killed in an 18-month-old uprising-turned-civil war.Syrian forces shelled rebel strongholds in the eastern suburbs of the capital Damascus on Monday and launched air strikes on the town of Salqeen in the northern province of Idlib, killing at least 17 people, activists said.The Syrian Observatory for Human Rights, which maintains a network of activists across Syria, said at least five children had been killed in Salqeen, including three from the same family.Video footage purporting to be of the victims showed the bloodied bodies of two children and an infant laid on a white sheet.With government forces holding the large medieval citadel in the heart of the Old City, according to rebels, the fighting that has already claimed more than 30,000 lives across Syria seems certain to destroy more cultural treasures too."Rebels control more that 90 percent of the Old City now," said Ameer, an opposition activist working with rebel brigades.But he said they were struggling to hold their positions under heavy artillery fire.He said the rebels still held the Souk al-Madina, a covered market of 13 km (8 miles) of vaulted stone alleyways and carved wooden facades that was once a major tourist attraction.Fires that damaged or gutted more than 1,500 shops had been put out, Ameer said, but new fires had now broken out in the Old City's Zahrawi, Aqaba and Bab Al Nasr markets.Plumes of black smoke were rising from many districts, and gunfire could be heard.