A bigger crime ring than previously thought may have been behind the theft of seven paintings, including a Picasso and two Monets and worth 18 million euros, from a Dutch museum last year, a Romanian court said on Wednesday.

Thieves made off with the paintings from Rotterdam's Kunsthal museum in October in a brazen and meticulously planned operation that was one of the biggest such thefts in the Netherlands.

Romanian authorities have detained three men but prosecutors suspect that other people may be involved.

"The three men joined forces with other people ... in stealing seven very valuable paintings," Lucia Zaharia, spokeswoman of Bucharest's district 5 court, said.

Zaharia would not elaborate on the names or nationalities of the other suspects, but said the insurance value of the stolen artworks was around 18 million euros ($24 million).

The works stolen were Picasso's "Tête d'Arlequin", Matisse's "La Liseuse en Blanc et Jaune", Monet's "Waterloo Bridge, London" and "Charing Cross Bridge, London", Gauguin's "Femme devant une fenêtre ouverte", Meyer de Haan's "Autoportrait" and Lucien Freud's "Woman with Eyes Closed". ($1 = 0.7526 euros)