|Most of the fleeing migrants are from sub-Saharan Africa, UN officials said [AFP]
Aid agencies have called on NATO and the European Union to help migrants fleeing Libya, hundreds of whom have drowned while trying to escape the fighting there.
"We are very concerned about what is happening in the Mediterranean Sea, where we are learning of many deaths due to capsized boats," Melissa Fleming, spokeswoman of the United Nations refugee agency (UNHCR), told a news briefing on Tuesday.
"The full death toll unfortunately is unknown to us," she added.
In one of the latest tragedies, migrants who arrived on the Italian island of Lampedusa at the weekend told the International Organisation for Migration (IOM) they had seen a ship carrying between 500 to 600 people sinking near Tripoli late last week.
It is unclear how many of those drowned.
"The tragic truth is we will probably never know how many people drowned in this latest tragedy," Jean-Philippe Chauzy, a spokesperson for the IOM, said on Tuesday. "That is the reality of the crisis in Libya."
Even before that incident, Fleming said the UNHCR believed that from March 25, at least 800 people fleeing Libya did not make it safely to shore.
Most of the migrants are from sub-Saharan Africa, according to UN officials.
The UNHCR has also appealed to European countries to step up efforts to rescue people fleeing Libya in overloaded and unseaworthy boats.
Fleming said European authorities patrolling the Mediterranean should not wait to receive distress calls from stricken vessels.
All vessels in the busy waters must be on the lookout for overcrowded boats and immediately raise the alarm that they are probably in distress, the IOM and UNHCR said.
"Any boat that is leaving Libya should be considered, at first glance, as a boat in need of assistance," Fleming said.
"We are reiterating our appeal to European states to urgently put in place more reliable and effective mechanisms for rescue at sea on the Mediterranean."
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Five boats carrying 2,400 people have arrived at Lampedusa in recent days, and every one of those boats needed to be rescued by the Italian coast guard and police, Fleming said.
The Italian and Maltese coast guards cannot carry the burden alone, she added, saying that there would be "more and more boats in distress".
"We honestly believe that the Italian coast guard is doing its best," Fleming said. But she added that, given how many people have drowned, "something isn't working".
Some migrants also said they had been forced on to boats by Libyan soldiers who fired warning shots and others said that although they had not officially had to pay for their crossing, they had been stripped of their possessions and savings.
NATO on Monday denied a report that alliance units failed to help a drifting boat carrying African migrants from Libya, leading to the deaths of 62 people from thirst and hunger.
France also denied that its aircraft carrier Charles de Gaulle was involved in the incident.
Guardian, the British newspaper, said the boat carrying 72 people, including several women, children and political refugees, ran into trouble after leaving Tripoli for the Italian island of Lampedusa 290km to the northwest on March 25.
It said that despite the alarm being raised with the Italian coastguard and the boat making contact with a military helicopter and a NATO warship, no rescue effort was attempted.
All but 10 of those who had been on board died after their vessel was left to drift in open waters for 16 days, it said.
The paper said it had concluded after extensive inquiries that the "NATO warship" it referred to was likely to have been the Charles de Gaulle, which was operating in the Mediterranean.
The French ship has been taking part in the international operation off Libya, but not under NATO command.
Thierry Burkhard, spokesman for the chief of staff of the French armed forces, denied the French navy had failed to help.
"The Charles de Gaulle was never at any moment in contact with this type of boat, nor (was) any other French vessel, due to its position," he said.
The Council of Europe (COE), the highest human rights body on the continent, called for an inquiry into the alleged failure to rescue the migrants in what it called a "dark day" for Europe.
"If this grave accusation is true – that, despite the alarm being raised, and despite the fact that this boat, fleeing Libya, had been located by armed forces operating in the Mediterranean, no attempt was made to rescue the 72 passengers aboard, then it is a dark day for Europe as a whole," Mevlüt Cavusoglu, the president of the council's parliamentary assembly, said.
"I call for an immediate and comprehensive inquiry into the circumstances of the deaths of the 61 people who perished - one by one - of starvation and thirst while Europe looked on."