Archaeologist Hermann Parzinger, head of the Prussian Cultural Heritage Foundation, was speaking on the eve of a two-day Berlin conference on ways to protect heritage sites in war-ravaged Syria.
Speaking to media, Parzinger said that Syrian troops, when they are off-duty, "are conducting illegal excavations" and "have looted" at the UNESCO World Heritage site.
Syrian troops backed by Russian air strikes and special forces recaptured Palmyra from the Islamic State (IS) group in March, delivering a major propaganda coup for both Damascus and Moscow.
IS jihadists had staged mass executions in the Roman amphitheatre, blown up ancient temples and looted relics in the one-time 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, the former head of the German Archaeological Institute.
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He said retaking Palmyra was "an important victory for culture," writing earlier in the Frankfurter Allgemeine Zeitung daily.
"But this victory has not made Assad and his backers the saviours of cultural heritage," he added.
"Assad's soldiers too plundered the ruins of Palmyra before the IS 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 over 170 scientists, archaeologists, architects and planners to discuss how to preserve Syria's heritage despite the five-year-old war that has killed more than 270,000 people.
UNESCO chief Irina Bokova, writing in the Tagesspiegel daily, wrote that "two-thirds of the old town of Aleppo have been bombed and burned" and at other sites, gangs have looted on an "industrial scale".
"Archaeological sites are in the crossfire ... and being misused as military bases," she wrote, while "Palmyra, which had long been insufficiently protected, has experienced indescribable horror and destruction" under IS control.