Burglars stole millions of euros' worth of jewelry from the museum of French luxury glassmaker Lalique in a daring early-morning raid Sunday, just months after a stunning gem heist at the Louvre in Paris.

A gang of masked thieves broke into the museum in Wingen-sur-Moder in northeastern France around 5:30 a.m. (3:30 a.m GMT), forcing a door before heading for the jewelry room and smashing open six display cases, a source close to the investigation told Agence France-Presse (AFP).

"Around 20 pieces of jewelry were stolen. The loss is currently being assessed but could amount to several million euros, likely close to 4 million," the source said.

Another source close to the investigation said the stolen jewelry was crystal, without precious gems, and could not be melted down.

The museum said on its website that it will be closed for several days because of the burglary.