(Changes dateline to Milan from Rome, writes through with news conference, arrest warrant)

MILAN, April 28 (Reuters) - Italian police have arrested four people suspected of conspiring to join Islamic State in the Middle East in a probe that revealed a plan to carry out a militant attack in Italy, a Milan prosecutor said on Thursday.

Italy has been spared deadly attacks by Islamist militant groups such those seen in recent months in France and Belgium, but authorities are nevertheless carrying out regular arrests of suspects, some of whom they accuse of plotting assaults.

As part of the same investigation, police also issued arrest warrants for two fugitives -- a Moroccan man and his Italian wife -- who left Italy and headed towards Iraq and Syria last year.

Investigators believe one of the suspects asked another to plan an attack in Italy and mentioned Rome, Milan prosecutor Maurizio Romanelli told a news conference.

"The new aspect here is that we are not talking about a generic indication (of an attack) but a specific person being appointed to act on Italian soil," Romanelli said.

"Rome attracts attention because it is a destination for Christian pilgrims," he said.

Last month, police in southern Italy arrested a 22-year-old Somali imam and asylum seeker on suspicion he was planning an attack in Rome.

The four arrested on Thursday were another couple living near Lake Como, a 23-year-old Moroccan man, and a female relative of the fugitive couple, police said.

The couple and the Moroccan man were planning to travel together to join Islamic State on its territory in Syria and Iraq, and the woman had helped put the two couples in contact with each other, police said.

The Moroccan man's brother was expelled from Italy last year on suspicion of having fought for the group, according to police.

(Reporting By Emilio Parodi, writing by Isla Binnie; Editing by Raissa Kasolowsky) ((isla.binnie@thomsonreuters.com; +39 06 8522 4392;))