Two of the survivors of the African boat tragedy
The UN Refugee Agency has expressed shock at reports that other vessels passed by and did not help as a boat sank carrying illegal immigrants.
About 75 illegal immigrants from Africa died while travelling on a crowded rubber dinghy between Libya and Italy.
It is thought many succumbed to hunger or thirst. Five Eritreans survived the journey and said no-one offered help.
The UN said the failure of other ships to stop and help represented a betrayal of maritime tradition.
Earlier this year, Italy and Libya began joint naval patrols in the Mediterranean to try to prevent the passage of illegal migrants.
Italian coastguards picked up the five survivors from the 12ft vessel, found drifting in Italian waters between Malta and the tiny island of Lampedusa.
The five - a woman, a child and three men - told rescuers that they had set out from the Libyan coast three weeks ago but had run out of food, water and fuel.
They said many people had died after drinking sea water and their bodies had been thrown overboard.
A spokesman for the UN Refugee Agency - the UNHCR - said the survivors had told them that a fishing boat which came across their stranded vessel offered them some bread and water, but then left them.
Other vessels simply passed by, the survivors reported.
"Apart from the shocking tragedy this represents it gives the UNHCR cause for concern that these people report being passed by many vessels without any assistance being offered.
"This is contrary to the long-standing maritime tradition of rescue at sea which has been under threat and is increasingly being eroded.
"UNHCR would be very concerned if the hardening of government policies towards boat people has the effect of discouraging ship masters from continuing to honour their international maritime obligations."
Many African migrants from across the continent gather in Libya to make the crossing to Europe, with hundreds of arriving every month on the island of Lampedusa.
From there they are taken by officials to detention centres on the Italian mainland, for identification and eventual expulsion.
Italy has recently introduced new legislation making it a crime to enter the country illegally, punishable by imprisonment and fines.
However, this doesn't appear to have created any great deterrent.
Hundreds of thousands still wait on the shores of north Africa for the chance to cross over to Europe, despite the serious risk to their lives.