"We have been warning for days, desperation has its limits," said Open Arms founder Oscar Camps, who released a video showing four migrants, in orange life vests, swimming toward Lampedusa island, while several crew members from the humanitarian group's ship swam quickly to catch up with them and bring them back aboard.

For days, Open Arms, which on Sunday had 107 migrants aboard, has been anchored off Lampedusa, a fishing and vacation island between Sicily and northern Africa. The boat initially had 147 migrants aboard when it reached Italian waters. In the last few days, 40 migrants have transferred by Italian coast guard vessels to Lampedusa, including a few who were ailing and 27 who said they were minors.

Right-wing Interior Minister Matteo Salvini has refused docking permission as part of his crackdown on charity rescue boats he contends essentially facilitate smuggling of migrants from traffickers' base in Libya.

Open Arms carried out its first rescue of this group 17 days earlier, plucking migrants to safety from smugglers' unseaworthy dinghies off Libya.

Seeking to break the standoff between Open Arms and Salvini, Spain on Sunday offered its southern port of Algeciras to Open Arms for disembarking, even while acknowledging that the destination is far and not suitable for disembarking.

The office of Spain's caretaker prime minister, Pedro Sánchez, said the offer reflected the "emergency situation" on the boat, and Salvini's "unconceivable response" in refusing docking.

"But right now, Spain is the only country ready to receive (the boat)," Sanchez' office said.

.But the offer appeared to have been made in vain.

The Spanish NGO's spokeswoman, Laura Lanuza, described the situation aboard the Open Arms as untenable.

"There is anxiety, bouts of violence, control is becoming increasingly difficult," she said. "To embark on a six day sailing with these people on board who are at the very limit of their possibilities would be crazy. We can't put their health and lives at risk."

Salvini earlier on Sunday told the shop to leave Italian waters and go to Spain and contended in a tweet that Open Arms was staying anchored off Lampedusa "just to provoke me and Italy."

Spain and five other countries have offered to take the migrants in, but Salvini hasn't let the ship dock.

Open Arms says Salvini is using the 107 migrants for "xenophobic and racist propaganda."