Islamic State (IS) fighters in Syria are said to have seized part of the town next to Palmyra, one of the Middle East's greatest archaeological sites.
Pro-government forces have evacuated local residents and are confronting the militants, Syrian state TV reported.
Activists say IS holds much of north Tadmur after overcoming militias loyal to President Bashar al-Assad.
Syria's head of antiquities said the world had a responsibility to save Palmyra, a Unesco World Heritage site.
Hundreds of statues had been moved to safety, but large monuments could not be moved, Maamoun Abdul Karim warned.
IS militants have ransacked and demolished several ancient sites that pre-date Islam in Iraq, including Hatra and Nimrud, leading to fears that it might attempt to damage or destroy Palmyra.