The Maltese Government is putting human lives at risk by illegally blocking rescue vessels, the NGO Sea-Watch said on Friday.

The NGO's vessel Sea-Watch 3 remains impounded in Malta as authorities investigate alleged registration irregularities. 

Its comment comes after migrant rescue ship Aquarius left Malta on Thursday after having disembarked 141 migrants the previous day. More than half of them were unaccompanied minors.

Watch: Aquarius reaches Malta, Italy agrees to take some of the migrants

Some feared the ship may be impounded in Malta - a fate already suffered by three other rescue vessels, including the Sea-Watch 3 - because of registration issues.

The MV Lifeline has been held up in Malta since June, after rescuing 233 migrants. Two other ships - the Seefuchs and the Sea Watch 3 – are reportedly facing questions over registration.

Sea-Watch is insisting that the latter has been prevented from sailing for 51 days despite fulfilling all conditions of the flag state and having all the necessary registrations.

It called on the government to put an “immediate end to the politically motivated blockade” of rescue vessels and stop "endangering human lives".

Read: 'I begged my captors to kill me': child migrants tell Aquarius crew they were kidnapped

It said that Dutch inspectors requested by Malta in July, have officially confirmed that the Sea-Watch 3 meets all the conditions of the Dutch flag state and that the necessary registrations are available.

To date, the Maltese authorities have not informed Sea-Watch of the requirements that must be met before they can leave port.

The NGO is concerned about the “systematic crackdown” on the rescue fleet as a whole.

Photo: Nick Jaussi/Sea-Watch

It believes that even if all rescue NGOs were operational, a scenario that currently seems unlikely, search-and-rescue capacity would remain at an alarming low level because of the withdrawal of European naval missions and the Italian government’s refusal of responsibility in maritime rescue coordination.

“The Maltese government is making up nebulous conditions that are not covered by any international law or regulation from the flag state that has already given the green light.

“While the civil rescue at sea is confronted with absurd accusations, the government, with the backing of the EU, is not concerned about law and order. This is a political campaign at the expense of people in distress,” Pia Klemp, captain of Sea-Watch 3 said.