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.

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.

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