Two of three stowaways who were rescued in Spain's Canary Islands after enduring 11 days on the rudder of a fuel tanker from Nigeria have been returned to the ship with the aim of deporting them.

The third person, who suffered hypothermia and dehydration during the voyage, has not yet been released from hospital on Gran Canaria, a local government spokesperson said. Under Spanish law, any stowaway who does not seek asylum must be returned by the operator of the ship to the port where the journey originated, a police spokesman told Reuters.

In a photograph on Twitter by the Spanish coast guard on Monday, the three stowaways are shown hunkered on the rudder under the hull, just above the waterline of the Alithini II.

The 183-metre ship, sailing under a Maltese flag, arrived in Las Palmas in Gran Canaria after setting out from Lagos in Nigeria on Nov. 17 and navigating up the West African coast, according to Marine Traffic.

The ship's captain confirmed to the Red Cross that it had sailed from Nigeria 11 days earlier.

A Canary Islands police spokesperson said it was up to the ship's operator to take care of the stowaways, provide them with temporary accommodation and return them to their origin as soon as possible.

The migrants should, at the least, have been informed of their right to ask for political asylum and should have been questioned before being returned to the ship, said Helena Maleno, director of the migration non-governmental organisation, Walking Borders. "The conditions of the journey are already an indication that something very serious may be behind it because the photos are incredible. We have never seen conditions like this where they have arrived alive," Maleno said.

She added: "These people have to be in a state of shock. They need a couple of days to recover and from there they can explain what they were running from to have made that decision."

Alithini II, owned by Gardenia Shiptrade SA, is managed by Athens-based Astra Ship Management, according to public shipping database Equasis.

Astra Ship Management did not respond to multiple calls from Reuters seeking comment. The Spanish government's representative on the Canary Islands didn't immediately respond to a query on whether the migrants should have been informed of their rights

The coast guard said the migrants were rescued by a coast guard vessel at about 7 p.m. local time (1900 GMT) on Monday.

The stowaways were treated for moderate dehydration and hypothermia, the Canary Islands emergency services and the Red Cross said. The third migrant, who was in a more serious state, had to be taken to a different hospital on the island.

The Spanish-owned Canary Islands are a popular but dangerous gateway for African migrants attempting to reach Europe. Since 2014, 2,976 migrants have died or are missing after attempting to cross from Africa to the archipelago by sea, according to the International Organization for Migration.

Reuters