Italy has threatened to take Brazil to the International court of Justice after the South American country released from prison a former communist guerrilla wanted by Rome in connection with four murders.

Cesare Battisti left a high-security Brazilian prison Thursday, a day after Brazil's Supreme Court rejected Italy's request for his extradition and ordered his release.

Italian Foreign Minister Franco Frattini said Italy plans to immediately activate every possible judicial mechanism to have Battisti extradited.

Prime Minister Silvio Berlusconi called the decision regretful and President Giorgio Napolitano said it contradicts the historical friendship between Brazil and Italy.

Italian prosecutors have charged Battisiti with four murders dating from the 1970s when he was a member of an armed communist movement. He denies the accusations.

The former left-wing militant had been on the run since the 1980s when he escaped from an Italian prison while awaiting trial. A court convicted him in absentia of two of the murders.

Battisti was arrested in Brazil in 2007 at the request of Interpol. Brazil granted him refugee status in 2009 on the basis that he could face political persecution if extradited.

Former Brazilian President Luiz Inacio Lula da Silva last year also denied Italy's request for his extradition.

Battisti faces possible life in prison if returned to Italy.