Italy threatened to break off relations with the John Paul Getty Museum of Los Angeles on Wednesday for failing to return pillaged art treasures.
The ultimatum was issued by Culture Minister and Deputy Premier Francesco Rutelli after the Getty suspended talks over the return of allegedly stolen artefacts now conserved in the museum.
"We have come to the end of the line with the Getty," Rutelli said.
"Either there's an agreement, with the return of all the works Italy has demanded, or we'll break off ties".
The dispute revolves around dozens of contested pieces, including two priceless antiquities - a bronze statue of an athlete attributed to the Greek sculptor Lysippos and a marble Venus found at the Ancient Greek colony of Morgantina near Enna, Sicily.
"We have shown that these works were stolen, illegally exported and then bought by the Getty," said Rutelli.
"We have negotiated for months with great patience. But now the time has come - the works stolen from Italy must be given back".
The Lysippos athlete, which the Californian museum acquired in 1977, was found in the Adriatic, off the northeastern port of Fano, in 1964.
The Getty claims that it was found in international waters and so does not belong to Italy.
Italy does not dispute that the bronze was outside territorial waters when it was discovered, but stresses that it was taken out of Italy illegally.
The Getty said it would only return the marble Venus - in co-ownership - if joint investigations established it had been plundered.
A former curator of the John Paul Getty Museum, Marion True, is on trial in Rome for allegedly acquiring stolen artefacts. It is the first such trial of an American museum curator.
Before the talks broke down the Getty, reputedly the world's richest museum, returned three of the allegedly stolen Italian treasures it had acquired.
On Wednesday Rutelli ruled out the hypothesis that Italy would settle for the return of only some of the contested pieces.
"That would not be an agreement, it would be a unilateral decision which the Italian government could not accept," he said Wednesday.
"They could decide to keep the works but they would come up against a very strong reaction from this ministry".
Earlier this year, Italy signed what some hope will be a groundbreaking deal with New York's Metropolitan Museum. This entails the return of artefacts in exchange for loans of equivalent value.
As well as the Met and the Getty, two other US museums with huge antiquities collections have come under the scrutiny of Italian investigators: Boston's Museum of Fine Arts and the Cleveland Museum of Art.
The Boston Museum has inked a similar deal to the Met's.
The February 21 agreement with the Met ended a 25-year wrangle over disputed antiquities.
The objects included one of the Met's gems, a sixth-century BC painted vase called the Euphronios Krater.
It is widely regarded as one of the finest examples of its kind.
Italian art police presented the Met with strong evidence that the red and black terracotta vase, or calyx, was stolen from the Etruscan burial site of Cerveteri near Rome in the early '70s.
The agreement also covered four ancient vases from Apulia (present-day Puglia) and a large collection of silverware stolen from Morgantina.
The vases will come home shortly, the Krater by January 15, 2008, and the silverware by January 15, 2010.
Met Director Philippe de Montebello has said he already has a "wish list" of loans he wants in exchange.
The Met deal and the True trial have signalled that Italy is getting serious about recovering lost artefacts.
By setting a precedent that could be used by other countries - notably Greece - Italy has sent alarm bells around the art world.
Rutelli said that "it is our government's duty to make it clear that all the world's museums which exhibit ransacked Italian works must return them".
A 1970 UNESCO convention, which both Italy and the US have signed, bans the import, export and transfer of ownership of cultural property.
