* Hundreds fleeing Libya believed drowned
* Adds to toll of thousands killed in fighting
* Aid agencies appeal to NATO, EU to help prevent more death
(Updates with NATO spokeswoman)
By Barbara Lewis
GENEVA, May 10 (Reuters) - Fleeing conflict in Libya has become almost as dangerous as staying behind to fight as hundreds of desperate migrants have drowned in a matter of weeks on unseaworthy vessels trying to cross the Mediterranean.
Aid agencies say witnesses have reported a vessel carrying between 500 and 600 people foundered late last week near the Libyan capital Tripoli. Many bodies were seen in the water, although it was unclear how many had drowned.
Before that, an estimated 800 people had gone missing from March 25 onwards after trying to escape from Libya, according to the U.N. High Commissioner for Refugees (UNHCR). Most of those trying to flee were from sub-Saharan Africa.
"The tragic truth is we will probably never know how many people drowned in this latest tragedy," Jean-Philippe Chauzy, a spokesman for the International Organization for Migration (IOM) said on Tuesday."That is the reality of the crisis in Libya."
The UNHCR also said the full toll was unknown, while other U.N. officials said they could not give precise numbers for the number killed in violence in Libya, which is thought to run into thousands.
Media have quoted Gene Cretz, the U.S. ambassador to Libya until the revolt started, as saying he had seen figures of between 10,000 and 30,000 but that it was impossible to know if they were accurate. Other estimates published earlier in the conflict have put the death toll from the fighting in the low thousands.
But future deaths could be prevented.
"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," UNHCR spokeswoman Melissa Fleming told a news briefing in Geneva.
"We have been in contact with officials at NATO... We have appealed to them at least to be aware that there will be more and more boats in distress."
The Italian and Maltese coast guards cannot carry the burden alone, Fleming added.
Many of those who have managed to cross the Mediterranean safely have landed on the tiny Italian island of Lampedusa or neighbouring Linosa.
The IOM said more than 10,000 had arrived on the islands since unrest erupted in northern Africa earlier this year, including roughly 2,000 last weekend alone.
Many were disorientated and confused, including one Somali woman, who managed to swim to back to the Libyan coast after escaping from the vessel which sank late last week. She lost her four-month-old baby and then was driven on to another boat.
Migrants 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 newspaper report alliance units had failed to help a drifting boat carrying African migrants from Libya, leading to the death of 62 people from thirst and hunger.
On Tuesday, a NATO spokeswoman in Brussels said ships under NATO command would always respond to calls from ships in distress.
"For example, I can tell you that on the 26th of March NATO ships responded to information that two migrant ships, with over 500 people on board were in difficulty.
"A NATO ship used its helicopter to airlift two women and a newborn child to medical help," Carmen Romero told a news briefing in Brussels, adding Italian authorities had then provided further assistance to the boats.
"So NATO is doing what it can to render assistance to those in distress at sea. And we will continue to do so."
Additional reporting by Stephanie Nebehay in Geneva, David Brunnstrom in Brussels and Christian Lowe in Algiers; editing by Michael Roddy