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The Spanish government said it is dispatching a naval ship to Italian waters to escort back a humanitarian group's boat with 83 migrants on board in deteriorating conditions.

In a statement, the office of Spain's caretaker center-left Socialist Prime Minister Pedro Sánchez, says the Audaz warship will end preparations and depart from the southern Spanish port of Rota on Tuesday afternoon.

The vessel's arrival in Lampedusa is not expected until Friday, the statement says, adding that it will then escort the Open Arms ship to the main port in the island of Mallorca.

The announcement came after 15 migrants jumped into the water in desperation to try and swim to Lampedusa after 19 days stuck on board.

They were "rescued and evacuated to Lampedusa," said a spokeswoman for the NGO Proactiva Open Arms that owns the ship.

The standoff with Italy will soon enter its third week, as Italy's hard-line interior minister Matteo Salvini refuses to allow the rescue ship access to a safe port.

There were initially 147 migrants on board but as the days have passed, some were evacuated for medical care and all minors were allowed to disembark.

European countries have agreed to take in the migrants, who were rescued at sea in early August of the coast of Libya.

But Salvini, who has plunged the Italian government into crisis by calling for fresh elections, has refused to allow migrant rescue vessels to dock as part of his hardline policies.

Spain on Monday stepped up criticism of Salvini's refusal, calling it "a disgrace to humanity".

"What Salvini is doing in relationship with the Open Arms is a disgrace to humanity as a whole," Spain's Defence Minister Margarita Robles told reporters in Madrid.

Salvini was "putting human lives at risk" for "exclusively electoral purposes", she added.

Separately, the organisation's founder, Oscar Camps, told The Associated Press in a phone interview on Monday night from Lampedusa, that "it is not a question of how long we can hold out. No one knows what will happen."

"At every instant we must stop fights, aggression, arguments, hunger strikes and stop people from jumping into the sea," exactly like several migrants did a day earlier in a foiled bid to reach the island, he said.

When the migrants were brought back aboard, major fights broke out among the passengers "because they had put the others at risk," Camps added.

Camps said Italy's coast guard offered to transport some of the migrants to Spain, but Open Arms insisted they must take everyone aboard.

Italian Transport Minister Danilo Toninelli, whose ministry includes the coast guard, said Italy also offered to escort Open Arms to a Spanish port, but the ship "incredibly refused" that offer, too.

Earlier in the day, Open Arms' president, Riccardo Gatti, suggested that the migrants could be transferred to the major Sicilian city of Catania, where a chartered plane could then fly the migrants to Spain.

Open Arms captain Marc Reig Creus told Italian authorities on Sunday that if Italy won't let the ship dock at Lampedusa, it would agree to transferring the migrants to another boat that could make the several days' journey to the port Spain had initially proposed, Algeciras, at the far west end of the Mediterranean.

Spanish Deputy Prime Minister Carmen Calvo said the government had offered to help the Open Arms with food, fuel and medical attention for the journey to Spain.

"We believe that once the migrants have peace of mind and know that they will navigate to a safe and open port, like the one Spain is offering, this situation will calm down," she said. "But the answer was that they (Open Arms) insist on entering Italy."

Camps, in explaining the refusal, said: "We could have done it on Day 1 or 2, but not on Day 18 when we have exhausted our resources," psychologically. "It can't be fixed with a little food, fuel and pats on the back" for a four-day journey, he said, calling conditions on the ship "inhuman."

Last week, 40 migrants and some family members were allowed to leave Open Arms because they were deemed to be minors, ailing or psychologically troubled.

Salvini wrote in a letter to Italy's Prime Minister Giuseppe Conte that he could authorise the "alleged" minors to leave the Open Arms ship despite such a move being "divergent to my orientation."

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