In the early hours of Tuesday, the thieves broke a window on the ground floor of the Van Buuren museum in the leafy Brussels suburb of Uccle and even though the alarm was triggered the thieves were gone before the police arrived.

"The whole thing only lasted two minutes and three seconds," said Isabelle Anspach, the curator of the museum.

The most expensive paintings were "The Thinker" by Dutch artist Kees van Dongen and "Shrimps and Shells" by Belgian painter James Ensor, which accounted for 80 per cent of the loss, Anspach said.

Belgian prosecutors said two witnesses saw the thieves speeding off in a BMW.