Around 40 people were missing off the Yemeni island of Socotra on Wednesday after a cargo vessel carrying islanders home from the mainland sank in the Indian Ocean, authorities said.

   Nineteen people were rescued from the water after a major search operation was launched in the early hours, Yemeni Fisheries Minister Fahd Kavieen told reporters.

   The first two survivors were rescued by a passing Austrian vessel and an Australian ship, the government's sabanew.com website reported.

   Kavieen did not give details of how the others were rescued.

   Although long ruled from Yemen, Socotra lies closer to the coast of Africa than it does to the Arabian Peninsula.

   The waters around the island lie at the exit of a busy shipping lane from the Mediterranean through the Suez Canal and the Red Sea into the Indian Ocean.

   For years it was prey to piracy from the lawless coast of nearby Somalia and it is now one of the most heavily patrolled sea areas in the world.

   Kavieen did not specify whether warships of the international counter-piracy operation were taking part in the search for survivors.

   He said that the ship had been missing for five days and its sinking had been confirmed on Tuesday.

   "The search is ongoing," he said. "Vessels have been combing the area since the early hours and there is significant hope that the passengers have survived."

   The Yemeni mainland has been ravaged by conflict for the past two years disrupting transport links to the archipelago.

   The port of Mukalla, from which the freighter departed for the islands, was controlled by Al-Qaeda for a full year until April.

   Air links to Socotra from elsewhere in Yemen have been virtually halted as the beleaguered government has battled rebels who still control the capital.

   There are no regular passenger ferries either, forcing islanders to seek berths on the occasional cargo vessel.

   Throughout the devastating conflict that has pitted forces loyal to President Abedrabbo Mansour Hadi against Shiite rebels and their allies, Socotra has remained loyal to his Saudi-backed government and has been spared the fighting.

   The island has enormous tourism potential which has never been realised because of the repeated conflicts that have gripped Yemen.

   Its isolation from the landmasses of both Africa and Asia has led to the evolution of unique plant life, much of it found nowhere else on earth.

   Among the most renowned is the dragon's blood tree, a bizarre umbrella-shaped species that earned its name from its blood-red sap which was much sought after as a dye in the ancient world.

   Persistent unrest in the nearby Horn of Africa has meant that the waters around Socotra have seen a steady flow of Ethiopian and Somali migrants ready to risk the perilous sea crossing in the hope of reaching Yemen's energy-rich Gulf neighbours.

   At least 79 people have perished while attempting to cross the Gulf of Aden this year, the UN refugee agency said. (AFP)