Suspect allegedly had maps of the synagogue's security on his computer [Reuters]

Italian police have arrested a Moroccan man on suspicion of plotting an attack against a synagogue in the city of Milan.

The chief suspect, identified only as a 20-year-old worker who had lived in Italy since age six, was arrested in the northern city of Brescia, 100km east of Milan.

The suspect was said to have had details of the synagogue and plans for an attack on his computer.

Meanwhile, in Britain, Scotland Yard detained a 40-year-old woman who had been in contact with the suspect to determine if she played any role in the alleged plot.

Anna Maria Cancellieri, the Italian interior minister, said that the matter was being treated "very seriously", although there was no terror alert in particular.

Lucio Carluccio, the head of Brescia police, said anti-terrorism investigators identified the suspect from internet traffic, including a Facebook page on how to assemble explosives and references in other contexts to a holy war.

The suspect had been under investigation for several months, but police arrested him after noting that he was using an internet application to zoom in on areas around the synagogue in a way that suggested he was examining the security, not browsing casually, said Carluccio.

'Particular alarm'

Authorities confiscated two flags emblazoned with Nazi swastikas at the suspect's residence, along with computers, USB drives, detailed descriptions of how to build a car bomb and the quantities of chemicals needed to make an explosive.

In Britain, Scotland Yard said the 40-year-old woman was detained in the early hours of Thursday on suspicion of collecting information useful to terrorism.

Authorities were searching the residence where she was arrested, and Scotland Yard was in contact with Italian authorities to determine if there was a link between the woman and the alleged plot.

Italy's interior minister, who is in charge of security services, played down any "particular alarm" about terrorism but said the arrest of the Moroccan was a "very serious activity".

The arrest warrant for the Moroccan was issued by authorities in Sardinia, who first identified the suspect in the course of another investigation.