Ancient statues and additional items have been taken from Syria's National Museum in Damascus, officials say.
The burglary was noticed on the start of the week, when staff reportedly found that an entrance had been damaged from the inside.
The six taken pieces were marble creations and originated to the Roman period, one official told the media outlet.
The nation's antiquities authority said it had initiated an inquiry to establish the "circumstances surrounding the loss of a number of items", and that steps had been enacted to improve safeguarding and monitoring systems.
The head of domestic security in Damascus province, Brig-Gen Osama Atkeh, was referenced by the official media as saying that security forces were probing the theft, which he said had focused on several "ancient sculptures and unique items".
He noted that museum protectors at the museum and other persons were being interviewed.
The cultural institution, which was created in the early twentieth century, contains the most important archaeological collection in the country.
It contains historical records tracing back to the ancient era from Ugarit, where evidence of the oldest known writing system was uncovered; Greco-Roman period ancient art from historical site, one of the most important historical locations of the classical era; and a ancient Jewish temple that was established at Dura Europos.
The museum was had to cease operations in 2012, twelve months after the outbreak of the devastating civil war. The majority of the holdings was evacuated and preserved at undisclosed sites to protect them.
It began limited operations in recent years and returned to normal in early this year, four weeks after rebel forces deposed the Assad regime.
All six of nationally recognized sites were affected or partially destroyed during the internal struggle.
The militant faction demolished several religious structures and historical sites at the ancient city, stating that they were un-Islamic. Unesco denounced the destruction as a war crime.
Numerous historical objects were also lost or looted from historical locations and museums.