More than 700 refugees are believed to have died in three shipwrecks in the Mediterranean, in what would be the deadliest week in the refugee crisis for more than a year.

The UN's refugee agency said scores of bodies had been recovered from the sea off the coast of Libya in the last few days, representing only a fraction of those feared missing.

Carlotta Sami, a United Nations High CommisSioner for Refugees (UNHCR) spokeswoman, said around 100 people are thought to have drowned when a wooden smugglers' boat capsized on Wednesday, an incident captured in dramatic photos by the Italian Navy.

Another 550 are feared dead after a boat carrying 670 capsized on Thursday morning, having left the Libyan port of Sabratha a day earlier, Ms Sami said.

According to survivors, the boat didn't have an engine and was being towed. Some 25 people survived by swimming to that second boat, while another 79 were recovered by members of the international mission in the Mediterranean.

Ms Sami said a third shipwreck occurred on Friday, during which 135 people were rescued. At least 45 bodies were recovered, taking the overall toll up to 700 – but survivors say many more are missing still.

If confirmed, the shipwrecks would account for the largest loss of life in the Mediterranean since April 2015, when the capsizing of a single smugglers’ boat is believed to have killed 800 people trapped inside.

Some 600 survivors arrived in Calabria on Italy's south-west coast on Sunday morning, while others were due to be taken to the mainland port of Taranto and to Pozzallo on the island of Sicily.

The UNHCR's update provides new clarity about Thursday's deadliest sinking, where initial reports only took into account the missing and dead from the smaller, powered boat.

Italian police have since corroborated the UN account, after their own interviews with survivors, though with slightly different numbers.

They say the boat being towed was carrying about 500 when it starting taking on water. Efforts to bail it out, with a line of passengers handing along 5-litre buckets, were insufficient and the boat was completely under water after an hour and a half, police said. At that point, the commander of the first smuggling boat ordered the tow rope to be cut.

“There were many women and children on board,“ Giovanna Di Benedetto, a spokeswoman for Save the Children, told Reuters. "We collected testimony from several of those rescued from both (the rubber and fishing) boats. They all say they saw the same thing."

Refugee crisis - in pictures Show all 27 1 /27 Refugee crisis - in pictures Refugee crisis - in pictures A child looks through the fence at the Moria detention camp for migrants and refugees at the island of Lesbos on May 24, 2016. AFP/Getty Images Refugee crisis - in pictures Ahmad Zarour, 32, from Syria, reacts after his rescue by MOAS (Migrant Offshore Aid Station) while attempting to reach the Greek island of Agathonisi, Dodecanese, southeastern Agean Sea Refugee crisis - in pictures Syrian migrants holding life vests gather onto a pebble beach in the Yesil liman district of Canakkale, northwestern Turkey, after being stopped by Turkish police in their attempt to reach the Greek island of Lesbos on 29 January 2016. Getty Images Refugee crisis - in pictures Refugees flash the 'V for victory' sign during a demonstration as they block the Greek-Macedonian border Refugee crisis - in pictures Migrants have been braving sub zero temperatures as they cross the border from Macedonia into Serbia. Refugee crisis - in pictures A sinking boat is seen behind a Turkish gendarme off the coast of Canakkale's Bademli district on January 30, 2016. At least 33 migrants drowned on January 30 when their boat sank in the Aegean Sea while trying to cross from Turkey to Greece. Getty Images Refugee crisis - in pictures A general view of a shelter for migrants inside a hangar of the former Tempelhof airport in Berlin, Germany Refugee crisis - in pictures Refugees protest behind a fence against restrictions limiting passage at the Greek-Macedonian border, near Gevgelija. Since last week, Macedonia has restricted passage to northern Europe to only Syrians, Iraqis and Afghans who are considered war refugees. All other nationalities are deemed economic migrants and told to turn back. Macedonia has finished building a fence on its frontier with Greece becoming the latest country in Europe to build a border barrier aimed at checking the flow of refugees Refugee crisis - in pictures A father and his child wait after being caught by Turkish gendarme on 27 January 2016 at Canakkale's Kucukkuyu district Refugee crisis - in pictures Migrants make hand signals as they arrive into the southern Spanish port of Malaga on 27 January, 2016 after an inflatable boat carrying 55 Africans, seven of them women and six chidren, was rescued by the Spanish coast guard off the Spanish coast. Refugee crisis - in pictures A refugee holds two children as dozens arrive on an overcrowded boat on the Greek island of Lesbos Refugee crisis - in pictures A child, covered by emergency blankets, reacts as she arrives, with other refugees and migrants, on the Greek island of Lesbos, At least five migrants including three children, died after four boats sank between Turkey and Greece, as rescue workers searched the sea for dozens more, the Greek coastguard said Refugee crisis - in pictures Migrants wait under outside the Moria registration camp on the Lesbos. Over 400,000 people have landed on Greek islands from neighbouring Turkey since the beginning of the year Refugee crisis - in pictures The bodies of Christian refugees are buried separately from Muslim refugees at the Agios Panteleimonas cemetery in Mytilene, Lesbos Refugee crisis - in pictures Macedonian police officers control a crowd of refugees as they prepare to enter a camp after crossing the Greek border into Macedonia near Gevgelija Getty Images Refugee crisis - in pictures A refugee tries to force the entry to a camp as Macedonian police officers control a crowd after crossing the Greek border into Macedonia near Gevgelija Getty Images Refugee crisis - in pictures Refugees are seen aboard a Turkish fishing boat as they arrive on the Greek island of Lesbos after crossing a part of the Aegean Sea from the Turkish coast to Lesbos Reuters Refugee crisis - in pictures An elderly woman sings a lullaby to baby on a beach after arriving with other refugees on the Greek island of Lesbos after crossing the Aegean sea from Turkey Getty Images Refugee crisis - in pictures A man collapses as refugees make land from an overloaded rubber dinghy after crossing the Aegean see from Turkey, at the island of Lesbos EPA Refugee crisis - in pictures A girl reacts as refugees arrive by boat on the Greek island of Lesbos after crossing the Aegean sea from Turkey Getty Images Refugee crisis - in pictures Refugees make a show of hands as they queue after crossing the Greek border into Macedonia near Gevgelija Getty Images Refugee crisis - in pictures People help a wheelchair user board a train with others, heading towards Serbia, at the transit camp for refugees near the southern Macedonian town of Gevgelija AP Refugee crisis - in pictures Refugees board a train, after crossing the Greek-Macedonian border, near Gevgelija. Macedonia is a key transit country in the Balkans migration route into the EU, with thousands of asylum seekers - many of them from Syria, Afghanistan, Iraq and Somalia - entering the country every day Getty Images Refugee crisis - in pictures An aerial picture shows the "New Jungle" refugee camp where some 3,500 people live while they attempt to enter Britain, near the port of Calais, northern France Getty Images Refugee crisis - in pictures A Syrian girl reacts as she helped by a volunteer upon her arrival from Turkey on the Greek island of Lesbos, after having crossed the Aegean Sea EPA Refugee crisis - in pictures Refugees arrive by boat on the Greek island of Lesbos after crossing the Aegean sea from Turkey Getty Images Refugee crisis - in pictures Beds ready for use for migrants and refugees are prepared at a processing center on January 27, 2016 in Passau, Germany. The flow of migrants arriving in Passau has dropped to between 500 and 1,000 per day, down significantly from last November, when in the same region up to 6,000 migrants were arriving daily.

Police in Ragusa have detained a man suspected to be the captain of the larger boat, state news agency Ansa have reported.

The traffic of people crossing in unseaworthy boats from Libya to Italy's southern islands has increased in recent days as other routes have grown more difficult and the seas have become warmer and calmer.