Thieves in the German city of Dresden have broken into one of Europe’s largest collections of art treasures, making off with three sets of 18th-century jewellery of “immeasurable worth” what German media has described as the biggest such theft since the second world war.

The dramatic heist took place at dawn on Monday, after a fire broke out at an electrical distribution point nearby, deactivating the museum’s alarm and plunging the area into darkness. Despite the power cut, a surveillance camera kept working and filmed two men breaking into the Grüne Gewölbe (Green Vault) at Dresden’s Royal Palace.

Volker Lange, the head of Dresden police, said the thieves smashed a window and cut through a fence before approaching and breaking open a display cabinet in the Grüne Gewölbe’s Jewel Room in “a targeted manner”.

Officers were at the scene within minutes of being alerted to the robbery shortly before 5am local time, but the suspects had escaped. A burning car found in Dresden early on Monday may have been the getaway vehicle, police said. They have set up roadblocks on motorway approach roads around the city in an attempt to prevent the suspects from leaving, as reported by The Guardian.

German media reported the losses from the burglary could run into the high hundreds of millions of euros, but the director of Dresden’s state art collections, Marion Ackermann, said it was impossible to estimate the value of the items. “We cannot give a value because it is impossible to sell,” she said, appealing to the thieves not to break the collections into pieces. “The material value doesn’t reflect the historic meaning.”

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