"(Prime Minister Silvio) Berlusconi must come before parliament before he goes to the United States next Thursday and say that he is no longer acting as a vassal," said Francesco Rutelli, leader of the centre-left Olive Tree coalition.

"If that is not done, we will vote for the return of our soldiers," he told Repubblica newspaper.

Berlusconi, a top US ally, is heading to Washington on May 19, shortly before US President George W. Bush visits Italy in June.

Italy has 3,000 troops serving with the occupation forces in Iraq, third only to the United States and Britain, and three Italian nationals are currently being held hostage by an insurgent group in the country.

Olive Tree parties have abstained during previous parliamentary votes on Italy's mission in Iraq but Berlusconi and his right-wing allies command a strong majority in the assembly.

Berlusconi himself condemned the prisoner abuse in Iraq on Monday.

"I am deeply distressed by the humiliation and suffering inflicted by certain US soldiers on certain Iraqi prisoners," he said in a statement.

And his minister for European affairs, Rocco Buttiglione, said the leaders of the US forces in Iraq should resign.

Italian opposition leaders called on Monday for Bush's visit to Rome to be cancelled as a protest against the abuse and torture of prisoners in Iraq.

"The Italian government would do well to cancel the events of June 4 with Bush, as long as full light has not been shed on the torture affair," said Green Party leader Alfonso Pecoraro Scanio.

WAR.WIRE