Robbers made off with three priceless diamond sets from a state museum in Dresden, police and museum directors confirmed Monday, in what German media have described as the biggest art heist since World War Two.

The thieves broke into the Green Vault at Dresden's Royal Palace -- home to around 4,000 precious objects made of ivory, gold, silver and jewels -- after a power cut deactivated the alarm at dawn Monday.

The stolen items included three "priceless" sets of diamonds, the director of Dresden's state art collections Marion Ackermann told reporters at a press conference on Monday.

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Ackermann confirmed the sets included brilliant-cut diamonds which belonged to an 18th-century collection of jewellery assembled by the museum's founder.

"We are talking here about items of inestimable art historical and cultural-historical value," she said but declined to give an exact estimate of the financial damages.

"We cannot put an exact value on them because they are priceless."

Dirk Syndram, another director at the museum, said the sets amounted to "a kind of world heritage", totalling about 100 jewellery items.

Bild daily said the heist was "probably the biggest art theft since World War Two".

"Brutality"

At dawn on Monday, a fire had broken out at an electrical panel nearby, deactivating the museum's alarm as well as street lighting, police said, adding the investigations were ongoing to determine if there was a link to the robbery.

Despite the power cut, a surveillance camera kept working and filmed two men breaking in.

The thieves had smashed a window and cut through a fence before "approaching in a targeted manner a showcase, which they destroyed", head of Dresden police Volker Lange said.

Ackermann said she was "shocked by the brutality of the break-in."

Founded by Augustus the Strong, Elector of Saxony in 1723, the Green Vault is one of 12 museums which make up the famous Dresden State Art Collections.

One of the oldest museums in Europe, the Green Vault holds treasures including a 63.8-centimetre figure of a Moor studded with emeralds and a 547.71-carat sapphire gifted by Tsar Peter I of Russia.

The museum is now made up of two sections -- a historic part and a new part.

And its historic section, which contains around three-quarters of the museum's treasures, was the one broken into on Monday.

With a strict limit on the number of daily visitors, entrance to the historic vault can only be reserved in advance.

Exhibits are arranged into nine rooms, including an ivory room, a silver gilt room and the central "Hall of Treasures".

One of its most valuable pieces, the green diamond, is currently on loan to the Metropolitan Museum of Art in New York, where it is a headline attraction in the temporary exhibition "Making Marvels: Science and Splendor at the Courts of Europe".

After the Royal Palace suffered severe damage in World War Two, the Green Vault remained closed for decades before it was restored and re-opened in 2006.

'Hard-earned' treasures

For Saxony's state premier, the heist went beyond the value of the artefacts stolen.

"The treasures that are found in the Green Vault and the Dresden Royal palace were hard-earned by the people of Saxony over many centuries," Michael Kretschmer said.

"One cannot understand the history of our country, our state without the Green Vault and Saxony's State Art Collections."

In 2010, the museum hosted a meeting between Chancellor Angela Merkel and then president of the United States Barack Obama, on the latter's first state visit to Germany.

Monday's theft is the second high-profile heist in Germany in recent years, after a 100-kilogramme (220-pound), 24-carat giant gold coin was stolen from Berlin's Bode Museum in 2017.