Libya: Hundreds feared dead as migrant boat capsizes
Several hundred people are feared to have drowned off Libya, after a boat carrying some 600 refugees trying to reach Europe broke up at sea on Friday.
The UN's refugee agency said 16 bodies, including two babies, had been found.
The UNHCR has said all ships using the Mediterranean should be ready to assist such vessels, as thousands continue to flee North Africa in inadequate boats.
Nato has denied claims that its naval units left dozens of migrants to die aboard another boat in distress.
It said it was unaware of the plight of the boat, which reportedly was adrift for more than two weeks.
The Guardian newspaper said 61 of the 72 people on board the boat died of hunger or thirst, despite being spotted by a military helicopter and Nato ship.
'Extra vigilant' journey
The UNHCR said migrants arriving on the Italian island of Lampedusa had reported seeing the boat carrying some 600 people foundering shortly after leaving the port at Tripoli on Friday.
If confirmed, this would be one of the largest accidents so far involving migrants leaving North Africa for Europe, often in unseaworthy boats.
Many of the witnesses were relatives of people on board, spokeswoman Sybella Wilkes told the BBC, adding: "There were a lot of distressed people on the quayside."
Ms Wilkes said it appeared that hundreds of people were missing - bodies were seen floating in the sea and those of 16 people, including two babies, have been washed ashore.
She said it was unclear whether anyone was looking for the missing people, but that Nato was not involved in the operation.
The nationalities of the passengers was also not known, but many of those waiting in Lampedusa were Somali, she said.
The International Organisation for Migration (IOM) said it had spoken to one Somali woman who had been on board the ship with her four-month-old baby. The baby died and the mother swam to shore, where she boarded another boat heading to Europe.
Ms Wilkes said "hundreds if not more than a thousand people" have died making the "treacherous journey" in the past few months.
The agency has said all ships should be prepared to offer immediate help to any boat carrying migrants.
"We are calling on all ships in the Mediterranean - whether commercial, military or otherwise - to be extra vigilant and consider on sight that these boats are in distress and needing rescue," said the spokeswoman.
The Guardian reported on Monday that a small boat had left Tripoli on 25 March, hoping to make it to Italy but ran out of fuel and started drifting.
Eventually food and water ran out, too.
"Every morning we would wake up and find more bodies, which we would leave for 24 hours and then throw overboard," one passengers, Abu Kurke, told the paper.
On 26 March, the passengers made contact with a priest in Italy, Father Mussie Zerai, who often plays a key role assisting migrants who hit trouble.
He confirmed to the BBC that he had alerted Italian coastguards, who said they would take action. But he lost contact with the boat when its phone battery went dead.
Abu Kurke said that shortly afterwards a helicopter appeared and dropped bottles of water and packets of biscuits onto the boat - but that after that, no further help arrived.
At one point - on 29 or 30 March, the Guardian says - the boat drifted close to an aircraft carrier. Survivors contacted by the paper said two jets took off and flew low overhead, while the migrants held two starving babies aloft. But no effort was made to assist them.
The Guardian said its inquiries suggested the ship must have been the French aircraft carrier Charles de Gaulle.
However, Nato said in a statement: "Only one aircraft carrier was under Nato command on those dates, the Italian ship Garibaldi. Throughout the period in question, the Garibaldi was operating over 100 nautical miles out to sea."
"Therefore, any claims that a Nato aircraft carrier spotted and then ignored the vessel in distress are wrong."
The Nato statement referred to the fact that the boat was supposed to be in "an unspecified location between Tripoli and [the Italian island of] Lampedusa" - and not 100 miles out to sea.
Nato said its vessels were fully aware of their responsibilities to assist vessels in distress - and indeed had rescued more than 500 people in two incidents off Tripoli on 26-27 March.
But Laura Boldrini from UNHCR told the BBC World Service it was "scary and concerning" that a boat could drift for so long in the Mediterranean without being picked up by any of the numerous military and commercial vessels operating there.
"This thing shouldn't ever happen," she said.
Up to 30,000 people are thought to have made the journey from Libya and Tunisia to Italy so far this year, driven by civil unrest and enabled by a collapse in emigration controls.