Survivors brought to safety in the Italian ports of Taranto and Pozzallo told the UN's refugee agency (UNHCR) and Save the Children how their boat sank on Thursday morning after a high-seas drama which saw one woman decapitated.
"We'll never know the exact number, we'll never know their identities," said the UNHCR's spokeswoman Carlotta Sami. But multiple witness reports suggested over 500 people had drowned in that shipwreck alone.
With some 100 others missing after a boat sank Wednesday, and 45 bodies recovered from a wreck that happened Friday, the UNHCR said it feared up to 700 people had drowned in total this week.
Giovanna Di Benedetto, Save the Children's Sicily spokeswoman, told AFP that survivors of Thursday's wreck said around 1,100 people had set out from Libya on Wednesday in two fishing boats and a dinghy, before running into trouble.
"The first boat, carrying some 500 people, was reportedly towing the second, which was carrying another 500. But the second boat began to sink. Some people tried to swim to the first boat, others held onto the rope linking the vessels," she said.
According to the survivors, the first boat's Sudanese captain cut the rope, which snapped back and decapitated a woman. The second boat quickly sank, taking those packed tightly into the hold down with it.
The Sudanese captain was arrested on his arrival in Pozzallo along with three other suspected people traffickers.
Trapped and drowned
"We tried everything to stop the water, to bail it out of the boat," a Nigerian girl told cultural mediators, according to La Stampa daily.
"We used our hands, plastic glasses. For two hours we fought against the water but it was useless. It began to flood the boat, and those below deck had no chance. Woman, men, children, many children, were trapped, and drowned," she said.
Those who survived told mediators the dead included "around 40 children, including many newborns", La Repubblica daily said.
"I saw my mother and 11-year old sister die," Kidane from Eritrea, 13, told the aid organizations. "There were bodies everywhere."
A bout of good weather as summer arrives has kicked off a fresh stream of boats attempting to make the perilous crossing from Libya to Italy.
Italian news agency Ansa said more than 15 boats had been setting off every day for the past five days.
Italy's Interior Minister Angelino Alfano said Europe needed "a quick agreement with Libya and African countries" to halt the crisis.
The chaos in the Libya since the fall in 2011 of late dictator Moamer Kadhafi has been exploited by people traffickers.
Migrants interviewed by La Repubblica in Sicily told the newspaper a new "head trafficker" called Osama had taken control of departures from Libya's beaches and was offering "cut-price" deals of €400 for the boat journey to lure in new customers.
'Violence is growing'
Doctors Without Borders (MSF) doctor Paola Mazzoni warned that "the number of women arriving pregnant or with newborns appears to be on the rise, with many saying they have been raped in Libya".
As newly arrived migrants docked in the port of Palermo, including a pregnant minor who said she had been raped, MSF mediator Ahmad Al Rousan said:
"Violence is growing in Libya -- we see it in the marks left on these boys and girls."
Many of the recent arrivals, largely Eritreans and Somalians, said they had balked at the idea of boarding the rickety vessels and dinghies once they saw them, but were forced on in Libya at gunpoint.
European Parliament President Martin Schulz said in an interview with the Repubblica on Sunday that Italy's "migration compact" idea was "the best proposal so far" for stopping the boat crossings and preventing deaths.
Italy wants to persuade African countries to help close migrant routes to Europe and take back some of those arriving via Libya in exchange for increased aid and investment.
Germany has made it clear, however, that it is against one of the elements of Italy's plan: the issuing of EU-Africa bonds to finance it.
Italian President Sergio Mattarella said Sunday that "actions of solidarity and reception coordinated across the world" were needed, as well as "credible policies on activating legal channels for migration and repatriation".