DRESDEN, GERMANY - Details are beginning to emerge on how burglars stole three diamond jewellery sets from one of Europe's largest treasure collections, the Dresden Green Vault in eastern Germany, in what appears to have been a meticulously planned heist.

The thieves, who are on the run, removed part of an iron grill on a ground-floor window and then smashed the glass in order to get into the museum.

There is speculation that the thieves disabled the museum's alarm system by setting fire to a nearby electricity junction box.

Firefighters were called in to tackle the blaze at around 5am on Monday.

Two suspects were seen in CCTV footage committing the crime in the dark, but police said that more people may have been involved in the robbery.

A car found burning in Dresden early on Monday may have been used by the burglars to commit the robbery, according to the police.

Guards were on duty at the museum on the night of the crime.

Dozens of police cars are now at the scene and the Green Vault is now closed.

The stolen sets of jewellery are also reported to include some rubies, emeralds and sapphires. One estimate pegged the value of the stolen jewellery at S$1.5 billion.

"We are talking here of objects of immeasurable cultural value," museum director Dirk Syndram told a news conference earlier.

It would be impossible to sell such unique, identifiable items on the open market, added Marion Ackermann, director of museums in the surrounding state of Saxony.

"It would be a terrible thing," she said when asked whether the jewellery might be broken up or melted down. Its cultural value far outstripped any material value, she added.