ISIS fighters have seized control of around one-third of the ancient Syrian city of Palmyra, a monitoring group said on Wednesday.
NBC News was not able to independently confirm the Syrian Observatory for Human Rights' reports of ISIS' advance.
Palmyra is home to a UNESCO world heritage site and is famous for its 2,000-year-old ruins, including a Roman aqueduct and necropolises. Syria's antiquities chief said on Saturday that the militants would destroy the ancient ruins if they took control of the city.
The monitoring group said that clashes between ISIS fighters and Syrian government forces were ongoing Wednesday. Airstrikes were launched on ISIS positions and casualties were reported on both sides, it said.
Professor Maamoun Abdulkarim, Syria’s director general of Antiquities and Museums, warned on Saturday that if ISIS took control of Palmyra it could lead to a "real disaster towards Syria's history and wipe out its glorious past."
He also drew attention to a U.N. resolution passed in February that condemned the "destruction of cultural heritage in Iraq and Syria" by ISIS fighters and other militants.
"We will win this battle targeting our existence, history and civilization," he said. "We will spare neither efforts, knowledge nor intentions to protect and safeguard our cultural heritage."