The International Organization for Migration (IOM) said Tuesday there were 10 bodies among the more than 6,000 boat migrants who were rescued the previous day off the coast of Italy.
Late on Monday, the Italian coastguard said it coordinated 39 rescue operations, also involving the Italian navy, the EU missions Eunavformed and Frontex, and several non-governmental organizations, including Doctors Without Borders (MSF).
"Over 6,090 migrants rescued at sea yesterday between Libya and Italy. 10 corpses found," the IOM spokesman in Rome, Flavio Di Giacomo, wrote on Twitter.
One of the victims was a pregnant woman, MSF wrote in a statement, indicating that many of the operations "took place under dramatic conditions," as rescuers found migrants, including women and children, nearly drowned and with fuel burns.
According to ANSA news agency, two other pregnant women gave birth overnight on an Italian coastguard vessel. The women and the babies were said to be in good conditions. The Guardia Costiera was not immediately available to confirm.
More than 6,000 migrant rescues in single day is a near record. They took place on the third anniversary a major Mediterranean migrant accident, the death of 366 people in a shipwreck off the Italian island of Lampedusa.
In a message, Italian President Sergio Mattarella said the tragedy was still "an open wound" and said "all the intelligence, humanity, organizational capacity that we have at our disposal" were needed to tackle migration flows to Europe.
Greece also recorded migrant arrivals - more than 600 over the last four days, including 280 in the past 24 hours, the Greek refugee coordinating body said. One week earlier, 90 landings were detected in the same time period.
The situation on the Greek islands where migrants arrive remains tense. A total of 14,600 are currently on Lesbos, Chios, Kos, Leros and Samos, nearly double the number who can be accommodated in the islands' refugee camps.
Conflicts and clashes continue to occur in the overcrowded facilities. The situation is particularly critical on Chios, whose two migrant camps have a regular capacity of 1,000 but are hosting more than 4,000 people.
According to a deal agreed by the European Union and Turkey, migrants arriving in Greece - most of them Syrians - should be sent back to Turkey. But they have a right to file for asylum before they are returned, which complicates the process.
Last week the European Commission said only 578 people had been sent back since the agreement came into force in March.
Low staffing and the large number of applications have hindered Greek authorities' ability to process asylum bids and return migrants. Greek judges have repeatedly accepted asylum requests based on the notion that refugees in Turkey are not safe.
According to IOM data last updated Sunday, 302,976 sea migrants arrived in Europe in the year to date, including 132,069 in Italy and 167,074 in Greece. The Geneva-based organization also estimated that 3,502 died or went missing in Mediterranean crossings.