Three pygmy marmosets -- the world's smallest monkey species -- have been stolen from a zoo in the western German city of Dortmund, police said on Wednesday.

The theft occurred overnight Sunday to Monday, police said, adding that the intruders also stole two other monkeys "and then managed to escape without being seen."

Two of the stolen pygmy marmosets -- which measure less than 15 centimetres (six inches) in length -- were being used for breeding, the German news agency DPA said, adding that the zoo had decided to install 24-hour surveillance following the incident.

In May, two families of rare and fragile monkeys -- seven golden lion tamarins and 10 silver marmosets -- were stolen from a zoo in Beauval in central France.