Syrian refugees who survived after their boat capsized off Malta say they were fired on by warring trafficking gangs as they set out on their perilous journey from Libya, the UN refugee agency says.
Thirty one people were killed and more than 200 people were rescued after the boat capsized a week after another shipwreck left 369 dead, prompting Malta to warn the Mediterranean was turning into a cemetery.
The boat, carrying up to 400 migrants, mostly Syrians, left the Libyan port of Zwara on Thursday, just 60 kilometres from the Tunisian border.
Citing testimonies from some of those who survived the 10-hour crossing, the UNHCR, spoke of "several injured passengers", saying that the shots were fired "perhaps by militiamen who shot to kill".
Molhake Al Roarsan, 22, interviewed in Valletta by the newspaper La Stampa, said that three people were injured after being shot in the arms and legs.
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He said he thought the shootings were related to a dispute between different groups of traffickers.
"There was a furious fight, screaming on the radio and on the phone with someone who demanded that we return to land, but the captain did not stop," said the Syrian.
The newspaper Repubblica reported that the shots were fired by a "Libyan patrol which is probably part of a criminal gang".
News agency Ansa said witnesses spoke of two people being killed during the shootings.
"They were shooting in all directions, on board there was panic with people trying to protect each other," it quoted one of the survivors as saying.
Once they reached Malta, the Tunisian captain was arrested after being recognised by survivors, according to media reports.
Those who travelled - mostly Syrians but also Palestinians - had to pay $US1,000 ($A1,060) to travel.
Following this latest tragedy, the Maltese prime minister, Joseph Muscat, warned that "we are just building a cemetery within our Mediterranean Sea".
The twin tragedies have prompted the European Union to call for sea patrols to cope with the flood of migrants knocking on its doors.
