The migrant rescue ship Ocean Viking was detained on Wednesday over technical irregularities, Italy's coastguard said, a move blasted as "harassment" by the charity that runs the vessel.

The ship had been allowed to disembark at a Sicilian port on July 7, with 180 rescued migrants including Pakistanis onboard transferred to another boat for a two-week coronavirus quarantine.

Those two weeks ended on Tuesday -- but the next day the Ocean Viking was immobilised.

The Italian coastguard said in a statement that an inspection had revealed "several technical and operational irregularities" and the ship was under "administrative detention" until they were corrected.

The unspecified problems were "of a nature likely to compromise not only the safety of the ship and its crew, but also of the people who have been and could be recovered onboard", it added.

The coastguard also referred to "violations of regulations aimed at protecting marine environment".

SOS Mediterranee, which charters the Ocean Viking, released a furious statement condemning "a blatant administrative harassment manoeuvre aimed at impeding our lifesaving work".

"Over the past three months, the same argument over safety has been systematically used by Italian authorities to detain four NGO ships," Frederic Penard, director of operations at the organisation, said in the statement.

"Why wasn't safety more of a concern to maritime authorities when, earlier this month, the Ocean Viking had to wait 11 days for a port to be assigned?"

The migrants -- mainly from Bangladesh, Eritrea, North Africa and Pakistan -- were picked up in four separate rescues in the Mediterranean on June 25 and 30.

Their disembarkment initially came as a relief, after a tense few days on the ship marked by migrants jumping overboard, a suicide attempt and bouts of violence.

The Ocean Viking resumed rescue operations on June 22 after a three-month halt because of the COVID-19 pandemic.

More than 100,000 migrants tried to cross the Mediterranean last year with more than 1,200 dying in the attempt, according to the International Organization for Migration.

The arrival of summer and more favourable conditions at sea may lead to an increase in attempts to cross the Mediterranean with the hope of arriving in Europe.

According to Italian news agency ANSA, customs officials rescued 90 migrants off the island of Lampedusa on Wednesday.