After two years at sea in which it has saved nearly 30,000 people, the Aquarius rescue ship is charting a course for a head-on collision with European governments.

The bright orange boat is at the centre of a blazing international row after Italy and Malta refused to take in its 629 passengers picked up off Libya over the weekend, including pregnant women and seriously injured patients.

With a capacity of 500, though it has frequently crammed more onto its decks when plucking migrants from flimsy boats, the Aquarius is the biggest ship carrying out such operations in the Mediterranean.

Supporters dub it the "ambulance of the seas" for its humanitarian mission to save migrants from drowning, but critics say its work only encourages people to attempt the perilous crossing.

Chartered by French charity SOS Mediterranee, the Aquarius set sail in early 2016, after a year that saw more than a million people flee war and poverty in Africa and the Middle East to reach Europe by sea.

Since then, the work of the ship has at best been viewed by European officials with indifference, and at worst with open hostility.

Things came to a head this weekend when both Italy's populist new government and nearby Malta refused to open their ports to the Aquarius.

It is now heading to Valencia in eastern Spain, 1,500km away, after new Socialist premier Pedro Sanchez offered to take the migrants in.

But with SOS Mediterranee defiantly insisting it will continue its rescue work, the standoff appears set to repeat itself, not least as warm summer weather prompts more to make the crossing from war-wracked Libya.

"Our mission is to rescue people at sea," SOS Mediterranee general manager Sophie Beau told AFP.

"As long as there are people drowning", the charity will work to save them, she vowed.


Italy closes its doors

Based in the southern French port of Marseille, SOS Mediterranee relies on private donations for more than 90% of the €11,000 needed daily to fund its operations.

Like other NGOs mounting rescues, the French charity has complained that its work has become increasingly difficult.

The situation has become especially fraught since last year when Libyan coastguards, trained by the European Union, began pushing NGOs further back from its coasts.

"The rescues are more and more complicated, slower and slower, with lots of confusion," said rescue coordinator Nick Romaniuk.

"Every interaction with the Libyans is very tense."

But the charity has a new and more pressing problem: Italy's new government.

SOS Mediteranee has always dropped its passengers off on Italian shores, adding to an influx of 700,000 migrants from Africa the country has had to deal with since 2013. 

But the message from hardline new interior minister Matteo Salvini is clear.

"Saving lives is a duty, turning Italy into a huge refugee camp is not," he tweeted Sunday.


Libyan 'hell' 

Despite the mounting difficulties, SOS Mediteranee says it will do everything to stand by its "promise to save people" as envisioned by its founder, the German sailor Klaus Vogel.

Formed in 2015 by three teams in Germany, France and Italy, it today has a staff of 140 rescuers from 18 countries.

When it was founded, the NGO was mainly seeking to fill the gap left by the Italian coastguard when it ended its Mare Nostrum operation rescue operation.

But critics have increasingly accused the group of actively encouraging migrant crossings, since those leaving Libya know there is a good chance they will be rescued.

French far-right leader Marine Le Pen has branded rescue charities "accomplices of the people-trafficking mafia" which make millions of dollars packing migrants into boats headed for Europe.

But SOS Mediteranee president Francis Vallat said the alternative, leaving people to die at sea, was inhumane. 

"These people are not leaving looking for an El Dorado, they are fleeing hell," Mr Vallat said in reference to Libya, where African migrants are often subjected to horrific abuse.

He said the charity would abide by the rules, up to a point.

"We have a red line," he said. "We are a citizen organisation which respects authorities unless one day we are asked to take refugees back to the hell of Libya."