A 22-year-old Moroccan man, believed to be part of the hit squad that carried out the terror attack on the Bardo National Museum in Tunis that killed 21 tourists, entered Italy on a migrant boat, an Italian news agency reported.

ANSA reported that the Tunisia Museum attacker, who was on the run after two of his partners were gunned down by the Tunisian police, arrived along with 90 migrants.

"A Moroccan national, wanted internationally, was arrested yesterday evening [Monday, 18 May] in a town in the Milan region. The Tunisian authorities suspect him of having taken part in the Bardo attack," a police spokesman told AFP on Wednesday.

Italy's anti-terrorism unit DIGOS arrested him. An NBC report stated that the Bardo Museum attacker has been identified as Touil Abdel Majeed.

More details on the arrest are expected to emerge after a press briefing later in the day. Italian news outlets said the 22-year-old Moroccan national was arrested in the town of Gaggiano in northern Italy.

The attack on the Bardo Museum on 18 March was carried out by three terrorists in military uniform. The attackers took several hostages and killed 21 tourists, mostly Europeans.

Two of the gunmen, Tunisian citizens Yassine Labidi and Saber Khachnaoui, were killed by the police. The third suspect was said to be on the run.

Initially, the Islamic State (Isis) had claimed responsibility for the attack. However, the Tunisian government blamed a local splinter group of Al-Qaeda in the Islamic Maghreb, called the Okba Ibn Nafaa Brigade, for the terror attack.