A spokesperson for the United Nations migration agency said more than 193 drowned over the weekend, and that those rescued showed signs of torture.
The Libyan Red Crescent said it had recovered 11 bodies on the shore in Zawiya.
"All the bodies are of female victims and there is a girl of less than one year old," said spokesman Mohanad Krima.
Some 50 survivors were picked up by a Danish container ship, the Alexander Maersk, which was alerted to divert by the Italian coastguard and dropped them off on Sunday in Pozzallo, southern Sicily.
Representatives of the UN High Commissioner for Refugees (UNHCR) and the International Organization for Migration (IOM) were able to meet them on Monday to hear their accounts.
Survivors told them that women and children were among those missing.
In all more than 6,000 asylum seekers were rescued Friday and Saturday in international waters off the coast of Libya and brought to Italy, while several hundred were rescued in Libyan waters and taken back to Libya.
According to the UN Refugee Agency, more than 1,150 people have died or disappeared since the start of the year while trying to reach European shores from North Africa.
"The increasing number of passengers on board vessels used by traffickers with an average of 100 to 150 people, are alarming and the main causes of shipwrecks," said Filippo Grandi, chief of the United Nations High Commission for Refugees.
"Risks are increased by the worsening quality of vessels and the increasing use of rubber boats instead of wooden ones."
A dangerous route
The sea channel between Italy and Libya is the world's busiest and most dangerous sea migration route.
A breakdown of law and order in northern Africa has allowed people-smuggling networks to operate with impunity. Smugglers extort money and hold people for ransom, and asylum seekers are often made to perform forced labour or sold in Libya's "slave" markets.
The number of people leaving from Libya in the hope of starting a new life in Europe is up nearly 50 percent this year compared with the opening months of 2016.
With most departures coming in the warm summer months, the trend points to around 250,000 people arriving over the course of 2017.
Some 500,000 asylum seekers were registered in Italy in the three years spanning 2014-16.
Italy and Libya have moved to boost cooperation in recent months in an attempt to curb the issue.
In February, the Italian government signed a deal with Libya and stated that it would offer manpower and technical assistance to the Libyan coastguard.
Source: News agencies