BERLIN- Six German men accused of involvement in a 2019 jewel heist at a museum housing one of Europe's greatest art collections appeared in court in Dresden on Friday, with the whereabouts of the treasures still a mystery.

The defendants, aged between 22 and 27 who were not named under German privacy laws, are charged with aggravated gang theft and serious arson, according to the Dresden public prosecutor's office.

They are suspected of breaking into Dresden's Gruenes Gewoelbe (Green Vault) Museum in the early hours of Nov. 25, 2019, and stealing 21 pieces of jewellery containing more than 4,300 diamonds with an estimated value of more than 113 million euros ($125.79 million).

"The defendants allegedly prepared the crime meticulously," Thomas Ziegler, a spokesman for Dresden district court, said.

Prosecutors believe that the six had checked out the crime scene beforehand, sawn through part of a window grating in advance and reattached it to get into the building as quickly as possible during the heist, Ziegler said.

Prosecutors said in September the defendants had not shed any light on the charges against them. Police offered 500,000 euros ($556,600) as a reward for anyone who could give information on the jewels' whereabouts.

Court documents show that the six defendants all have the same last name. The trial is expected to continue until the end of October. More than a dozen defence lawyers are representing the suspects who are in custody.

Two have already been sentenced to four and a half years in prison for involvement in stealing the Big Maple Leaf, a 100-kg gold coin worth 3.75 million euros, from Berlin's Bode Museum in 2017.

The stolen Dresden collection was assembled in the 18th century by Augustus the Strong, Elector of Saxony and later King of Poland, who commissioned ever more brilliant jewellery as part of his rivalry with France's King Louis XIV.

The treasure survived Allied bombing raids in World War Two, only to be carted off as war booty by the Soviet Union. They were returned to Dresden, the historic capital of the state of Saxony, in 1958.

(Reporting by Riham Alkousaa in Berlin, Leon Malherbe and Fanny Brodersen in Dresden Editing by Mark Heinrich and Nick Macfie) ((Riham.Alkousaa@thomsonreuters.com;))