More than 50 people attempting to get from Libya to Italy in a rubber boat have died of thirst.
The United Nations Refugee Agency (UNHCR) said a sole survivor of the 15-day voyage told of how 54 people died as their dinghy slowly deflated.
The man was spotted clinging to the remains of the boat by fishermen and was rescued by Tunisian coastguards.
The Eritrean national was found in an advanced state of dehydration and taken to a hospital in Zarzis, Tunisia.
He told officials that a group of 55 people fled Libya's capital Tripoli at the end of June and almost made it to the Italian coastline a day later.
But strong winds forced the boat back out to sea, and after a few days it had ruptured and began to deflate.
There was no fresh water on board, and some passengers resorted to drinking sea water which only made them thirstier, the man said.
People began to die within days, including members of his family, he added.
More than half of the victims are thought to have been from Eritrea, and the remaining victims from different countries.
UN spokeswoman Laura Boldrini said: "There were immediately problems on the boat - unfortunately they weren't even allowed to take a bottle of water and so once they got lost and the voyage went on, people started to feel unwell and die because of the lack of water."
"How is it possible that in a sea like the Mediterranean which is full of fishing boats, cargo ships, naval ships, these people were just left to their fate like that?" she asked.
Another spokeswoman described the man as being in "a pretty awful state" after the "terrible experience progressively watching his family members dying".
There are often disasters when migrants attempt to reach southern Europe from North Africa in unsuitable vessels.
Around 170 people have died at sea this year trying to reach Europe from Libya, the UNHCR said.
Greece and Italy are the two main entry points for undocumented migrants, and both countries have seen an increase in arrivals following last year's Arab Spring revolts.
Since the beginning of 2012, around 1,300 are thought to have reached Italy by sea and another 1,000 people have landed in Malta.
The busiest period for crossings is between May and September, when the Mediterranean Sea is at its calmest, the agency said.