Hermann Parzinger, president of the Prussian Cultural Heritage Foundation, said off-duty soldiers were conducting illegal excavations and had looted at the Unesco world heritage site.
Parzinger was speaking on the eve of a two-day conference in Berlin on ways to protect heritage sites in war-ravaged Syria.
Syrian troops backed by Russian airstrikes and special forces recaptured Palmyrafrom Isis in March, delivering a major propaganda coup for Damascus and Moscow.
Isis militants had staged mass executions in the Roman amphitheatre, blown up ancient temples and looted relics in the former trade hub in central Syria, a major tourist site before the war.
Despite the liberation, “we shouldn’t act like everything is alright now,” said Parzinger, former president of the German Archaeological Institute.
He wrote in the Frankfurter Allgemeine Zeitung daily that retaking Palmyra was “an important victory for culture … But this victory has not made Bashar al-Assad and his backers the saviours of cultural heritage.
“Assad’s soldiers too plundered the ruins of Palmyra before the Isis takeover, and their rockets and grenades indiscriminately pounded the antique columns and walls when this promised even the slightest military advantage.”
From Thursday, the German government and Unesco will host more than 170 scientists, archaeologists, architects and planners to discuss how to preserve Syria’s heritage despite the five-year war that has killed more than 270,000 people.
Unesco’s director general, Irina Bokova wrote in the Tagesspiegel daily that two-thirds of the old town of Aleppo had been bombed and burned, and at other sites gangs had looted on an “industrial scale”.
“Archaeological sites are in the crossfire … and being misused as military bases” while “Palmyra, which had long been insufficiently protected, has experienced indescribable horror and destruction” under Isis control, she wrote.