The migrant rescue ship Sea Watch 3 has been detained in the Sicilian port of Catania for breaching safety and environmental laws, the Italian coast guard said Friday.

The crew of the Dutch-flagged ship had expected the vessel to be detained or impounded as a show of strength from Italy's far-right Interior Minister Matteo Salvini, who says charity rescue boats aid illegal migration.

"As assumed, we are blocked for political reasons! There was high pressure on the coast guard to find a reason to chain us," Sea Watch International said on Twitter.

"They can chain our ships, but they can´t chain solidarity!" it said.

Officials boarded the ship on Thursday after it disembarked 47 migrants rescued in the Mediterranean "to perform a technical inspection of conditions on board", the coast guard said.

"During the inspection of the Sea Watch 3, a vessel certified as pleasure yacht, a series of non-compliances concerning both navigation safety and compliance with rules protecting the marine environment were noted," it said.

The coast guard will "not permit the departure of the ship until they are resolved," it added, without providing further details.

Sea Watch's mission head Kim Heaton-Heather told AFPTV on Thursday that he feared "problems with the authorities".

"But I am also very, very certain that in the end, no matter what allegations are brought against the organisation, the ship or the crew as whole, none of these allegations will stick and the truth of the matter will come out," he said.

Salvini, who is looking into whether it is possible to ban charity rescue ships from Italian waters, accuses the Sea Watch crew of sailing straight for Italy rather than taking the migrants to closer ports in Libya or Tunisia.

The German charity says it tried but failed to get a response from Tripoli or Tunis and had no other option than to head to Italy.

No more civilian rescue boats off Libya

Sea Watch 3 is the latest in a long list of such ships that have been forced or have chosen to stop their work.

Currently only the Libyan coastguard is able to save those migrants floundering in their attempts to reach Europe across the central Mediterranean.

Here is a status review of migrant rescue ship organisations:

Gave up

The main charities trying to rescue migrants off the coast of Libya stopped their efforts in the summer of 2017, after departure numbers fell and the Libyan coastguard intensified threats against the boats they consider accomplices of people smugglers.

Maltese aid group MOAS, which was the first to carry out migrant rescue operations in 2014 and had deployed two vessels, transferred its activities to helping the Rohingya in Bangladesh in September 2017.

Around the same time, Doctors Without Borders (MSF) ended its operations with the Vos Prudence, the largest private vessel deployed off Libya with a record 1,500 people rescued at the same time.

Save the Children ended its search and rescue operations with the Vos Hestia in October 2017.

Stopped

In August 2017, Italian authorities impounded the Juventa, operated by small German aid group Jugend Rettet, after it was accused of helping Libyan human traffickers. The aid group denies the charge.

The Lifeline rescue vessel operated by a German aid group of the same name was impounded on arrival in Valletta, Malta, in June 2018, for alleged registration issues.

Aid groups SOS Mediterranee and MSF stopped search and rescue operations with the Aquarius in December after it was stuck in a French port for two months following the revocation of its registration.

Spain's Proactiva Open Arms in January slammed the authorities' decision to keep the Open Arms ship in Barcelona harbour. The boat was impounded for a month by Italy in early 2018. It was then forced to take rescued migrants to Spain several times after Malta and Italy refused to allow them to disembark.

Persevering

The Sea Eye charity from Germany had several vessels impounded during 2018 but deployed another ship, the Professor Albrecht Penck, in December, rescuing 12 migrants. The boat is currently in Majorca and plans to set sail again in around two weeks.

SOS Mediteranee has said it is looking for another boat and flag so it can continue search and rescue operations.

In Italy a collective of associations launched the Mediterranea, flying an Italian flag, mainly to witness the situation for migrants off Libya.

There are also two light aircraft, the Colibri operated by French aid group Pilotes Volontaires and the Moonbird operated by Sea Watch, which overfly the Mediterranean seeking to identify and locate migrant-carrying boats in trouble.