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By Cassandra Vinograd

ISIS said it was in full control of the ancient city of Palmyra on Thursday following a bloody battle with Syrian pro-government forces, a gain rife with strategic as well as historical implications.

In a statement labeled "Breaking News," ISIS announced the victory in Palmyra and said its fighters were in control of the "notorious" nearby prison and airport. The statement — released by the ISIS authority in Homs and distributed by the group's supporters on Twitter — said that retreating pro-government forces had left behind "large numbers" of their dead.

Syrian state TV acknowledged that pro-government forces had withdrawn from Palmyra — marking what appeared to be the first time ISIS had directly seized a city from forces loyal to President Bashar Assad.

The capture of Palmyra gives the militants as much of a strategic gem as a cultural one. While the city is home to a UNESCO world heritage site and famous for its 2,000-year-old ruins — there are many other prizes in line for ISIS to potentially plunder.

“There’s a lot of strategic assets in the area which ISIS will be gunning for — military bases, weapons depots, oil and gas facilities,” said Matthew Henman, head of IHS Jane’s Terrorism and Insurgency Center. “For ISIS, whenever moving in, territory is key but the thing it really prioritizes is strategic assets.”

ISIS now controls more than 50 percent of Syria — or over 36,000 square miles — with the capture of the Palmyra, according to the Syrian Observatory for Human Rights, an activist monitoring group.

While much of that territory is sparsely-populated empty desert in the east of the country, Henman said the capture of Palmyra still has significantly expanded its territorial control by overrunning Palmyra and put the already-stretched Syrian government forces under greater pressure, Henman said.

“Things aren’t going to get any better for the government,” Henman said. “It’s pretty bad news.”

Concerns continued to mount that ISIS militants would set their sights on destroying Palmyra's irreplaceable and historic ruins, including a Roman aqueduct and necropolises.

ISIS already has raided and destroyed priceless antiquities in neighboring Iraq, from laying waste to the 3,000-year-old city of Nimrud to smashing relics in a Mosul museum.

UNESCO's Secretary General Irina Bokova has repeatedly called for an end to hostilities in Palmyra, saying Wednesday that the fighting "is putting at risk one of the most significant sites in the Middle East, and its civilian population."

NBC News' Alexander Smith contributed to this report.