One of the paintings stolen from a Brazilian museum was put up for sale on an internet auction site for $13m (£7.4m), according to police.
The art robbers disappeared in a crowd of Samba revellers
Henri Matisse's Luxembourg Gardens, which was taken from the Chacara do Ceu museum in Rio de Janeiro, was advertised on a Russian website.
Brazilian police say they believe the stolen pictures are still in Rio.
The picture was taken along with works by Picasso, Dali and Monet, while carnival festivities were under way.
Investigating officer Deuller Rocha said Interpol had joined in the widening search.
He said: "Given the characteristics of the crime, the way the thieves acted, it may be a contract job done for an international crime ring."
The Chacara do Ceu museum has put up a reward of $5,000 (£2,876) for the safe return of the paintings.
Experts say the artworks, the museum's most valuable pieces, are worth up to $50m (£28.7m).
Police have also issued computer-generated images of the armed thieves and given details of the paintings to 182 countries via Interpol, while Brazilian authorities are on alert to stop the items leaving the country.
At least four men are believed to have been involved in the heist last Friday, in which security guards at the museum were threatened with a hand grenade.
The thieves shut down the security cameras and then made their getaway during the carnival, said museum officials.
The other works include Pablo Picasso's The Dance, Salvador Dali's The Two Balconies and Claude Monet's Marine.
A number of tourists who were visiting the museum at the time were also mugged by the gang.