Zurich's prestigious Bührle Collection is re-opening to art lovers on Monday, more than six months after thieves stole four paintings during an armed attack.

The collection's director Lukas Gloor said on Thursday that security had been reinforced since the heist in February, which saw the robbers make off with four Impressionist works worth an estimated SFr180 million ($165 million).

Groups will be able to visit the collection but will have to announce their arrival at least six weeks in advance. Individuals will be able to join a guided tour the first Wednesday afternoon of each month.

Visiting hours could be extended at a later time.

Only two of the stolen paintings have been recovered, a Monet and a Van Gogh, which were found a few days later in a car outside a psychiatric clinic. The two others, a Cézanne and a Degas, have not been seen since the theft.

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