Unaccompanied youths who were among migrants stranded on the Spanish migrant rescue ship Open Arms are pictured before disembarking from an Italian Finance Police boat, in Lampedusa, Italy, on Saturday. [Photo/Agencies]

MADRID-Spain on Sunday offered to take in the charity vessel Proactiva Open Arms with more than 100 migrants on board, slamming Italy's "inconceivable" refusal to allow it to dock.

Spanish Prime Minister Pedro Sanchez "today ordered the port of Algeciras to be ready to receive the boat Open Arms", which is currently in limbo off the Italian island of Lampedusa, his office said in a statement.

It cited the "urgent situation" on the vessel and the "inconceivable decision by the Italian authorities ... to close all its ports" to the migrants.

Italy's far-right Interior Minister Matteo Salvini, who is also deputy prime minister, has refused to allow migrant rescue vessels to dock as part of his hard-line policies.

The Open Arms standoff with Italian authorities was the latest between Rome and charity vessels rescuing migrants-mainly sub-Saharan Africans-making the perilous journey across the Mediterranean Sea.

The Open Arms organization said Madrid had taken the decision because of the "untenable" situation for the 107 migrants still aboard, including two children.

"Spanish ports are not the closest or the safest for Open Arms, ... but for now Spain is the only country willing to accept it," the organization said.

Salvini, who leads the anti-immigrant League party, has taken a strick position against rescued migrants coming to Italy. He says his country bears an unfair burden as the first port of call for migrants.

France said on Sunday it would take 40 of the migrants on board the Open Arms as long as they meet the criteria to be treated as refugees.

"France commits to meet its promise to take 40 of the people," the interior ministry said.

European Union states disagree on how to handle the migrants crossing the Mediterranean and some, like Italy, are taking a tougher position, turning away rescue ships as they seek to dock to drop off rescued passengers.

Spain's Foreign Ministry on Sunday called again on Italian authorities to allow the migrants entry, guaranteeing that other EU states would take them in once they reach land.

"The Italian government can be assured that when they disembark, the 107 migrants will be shared" among countries willing to take them: France, Germany, Luxembourg, Portugal, Romania and Spain, it said.

Italy's government on Saturday reluctantly agreed to let 27 migrant children from the Open Arms disembark but it required other passengers to stay aboard.

The Open Arms has been anchored since Thursday within swimming distance of Lampedusa and crew say the situation on board is critical after two weeks at sea.

Open Arms founder Oscar Camps on Sunday published a video showing four migrants jumping off the ship in a desperate bid to swim ashore before they were stopped and brought back.

The International Organization for Migration says at least 840 people have gone missing so far this year while trying to cross from Libya to Europe.