Ocean's Eleven was a worldwide box office hit

Inquiries revealed that some Mafia suspects were seen loitering near the set, a police spokesman said.

They may have been looking to blackmail film-makers or were concerned about the discovery of an alcove used for arms.

The Ocean's Eleven follow-up, starring George Clooney, Brad Pitt and Julia Roberts, is due out in the UK in 2005.

The sequel also stars Catherine Zeta Jones and Elliott Gould.

Undercover

Some of the scenes for the crime caper have been shot this month in the seaside town of Scopello, western Sicily.

Police official Giuseppe Linares said the film's crew and cast were unaware of the undercover investigation into local Mafia activity.

He said authorities did not inform the film-makers of the secret surveillance. No crimes were recorded at the set.

News of the on-set surveillance emerged on Tuesday, as police in Sicily announced that they had arrested 19 alleged mobsters after a two-year investigation.

The gang's alleged crimes revolved primarily around the extortion of local businesses around the town of Castellammare del Golfo, near Trapani.

The 19 suspects and four others already in prison are accused of a range of crimes, from Mafia membership, to extortion, to the illegal possession of arms and explosives.