"The Audaz, which will leave this afternoon at five pm (1500 GMT), will sail for three days to Lampedusa where it will take charge of the people taken in by the Open Arms," the government said in a statement.

The patrol boat will escort the Open Arms charity ship back to the port of Palma in the Balearic Islands.

"After analysing various options, the Spanish government... considers this is the most suitable" for "the humanitarian emergency on board the Open Arms."

The announcement came after 15 migrants jumped into the water in desperation to try and swim to Lampedusa after 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 charity warned the situation was "out of control," with some of the migrants stuck for 19 days after being rescued at sea off the coast of Libya.

The vessel has been anchored since Thursday within swimming distance of Lampedusa, seeking permission to dock, with the situation increasingly tense.

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.

Six European Union countries -- France, Germany, Romania, Portugal, Spain and Luxembourg -- have offered to take in the migrants.

But Italy's far-right Interior Minister Matteo 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 slams Italy -

Faced with Italy's protracted refusal to allow the migrants to disembark, Spain at the weekend offered up its southern port of Algeciras, which the NGO said could "not be achieved" due to the distance and tensions on board.

Madrid then offered up Mallorca in the Balearic Islands, nearer but still around 1,000 kilometres (600 miles) from Lampedusa.

The charity described this offer as "totally incomprehensible" and continued to demand the ship be allowed to dock in Lampedusa.

Spain's Defence Minister Margarita Robles has slammed Italy -- and particularly Salvini -- for the situation.

"What Salvini is doing in relationship with the Open Arms is a disgrace to humanity as a whole," she said Monday.

Salvini "has shown he doesn't care about human lives", she added Tuesday.

Salvini tweeted that "being firm is the only way to stop Italy from becoming Europe's refugee camp again."