(CNN) At least 100 people are believed to have died after a boat carrying migrants sank off the coast of Tunisia on Saturday, the highest death toll in any single migration attempt this year, the International Organization for Migration (IOM) announced Tuesday.

By Tuesday morning, the Tunisian coast guard had recovered 51 bodies and rescued 68 people, according to the country's interior ministry, which estimates that 180 people were on board. The remaining missing people are presumed dead -- causing Monday's initial death toll of 48 to rise significantly.

The boat was 16 nautical miles off the Sfax coast -- 170 miles south of the capital Tunis -- when it sent a distress signal at 10:45 p.m. local time (5.45 p.m. ET) on Saturday, the interior ministry said.

Wael Ferjani, a Tunisian man who survived the wreck, told AFP that the boat's capacity was "75 to 90 people maximum, but we were between 180 and 190 on board."

"Water was coming into the boat," he said. "Those who could escape fled, others drowned. We stayed there until almost 5 a.m., then fishermen came to help us, and then the army arrived."

The migrants reportedly paid 2,000 to 3,000 Tunisian dinars ($780 to $1,170) for the crossing, according to Lorena Lando, IOM chief of mission in Tunisia. The "fragile and overloaded" fishing boat began taking on water two hours after leaving the coast of the Kerkennah Islands, near Sfax, IOM said Tuesday

Around 120 people in the boat were Tunisian, according to Lando. Several people from sub-Saharan Africa as well as Libyans and Moroccans are among the survivors. Some of the dead are yet to be identified.

"There are no words to describe this tragedy," said Lando. "Behind these numbers, men, women and children have lost their lives while pursuing an uncertain dream."

Another nine migrants, including six children, died after their boat sank off the southern coast of Turkey early Sunday morning, the Turkish coast guard said in a statement. Five others were rescued and at least one person is still missing.

People in Sfax wait for news of friends and relatives involved in the shipwreck.

'Massacre on the sea'

Although the number of people trying to reach Europe by boat from Turkey and northern Africa has fallen significantly in the past two years, the Mediterranean remains the world's deadliest migration route.

Since the beginning of 2018, around 785 people have died en route, compared to 87 at the US-Mexico border and 82 in sub-Saharan Africa, according to IOM data

Tunisia, which neighbors Libya, is frequently used as a starting point by migrants, who often make the journey in overcrowded, makeshift boats heading for Sicily, around 250 miles away.

Francesco Rocca, president of the International Federation of Red Cross and Red Crescent Societies (IFRC), said in a statement Tuesday there is "an urgent need for global answers to protect the lives and basic human dignity of people on the move."

"We cannot keep silent as the massacre on the sea continues," he said, adding: "The situation on the ground is not changing. On the contrary, it is worsening."

UN member states will meet in New York this weekend for the fifth round of negotiations on a new Global Compact for Migration. The IFRC is calling on governments to prioritize efforts to save lives and to ensure the agreement protects all migrants at all stages of their journeys from violence, abuse and other rights violations.

Violence and abuse for migrants returned to Libya

The falling numbers attempting the dangerous journey across the Mediterranean are ascribed partly to two controversial deals: one struck in March 2016 between the European Union and Turkey -- under which Syrian refugees arriving in Greece from Turkey are sent back -- and another made last year between Italy and Libya , in which the southern European country pledged to bolster Libya's coast guard so it could spot departing migrant boats and house migrants attempting to cross.

Italy's new interior minister and leader of the anti-immigrant League party, Matteo Salvini, said Sunday that the "best way" to save people's lives in the Mediterranean is "to prevent them from boarding those ships."

Aloys Vimard, project coordinator on the Aquarius, a search-and-rescue ship in the Mediterranean run by Doctors Without Borders and SOS Méditerranée, disagrees. "Containment and deterrence doesn't work," he told CNN. "It doesn't save lives -- it just encourages people to take bigger risks."

"Many of them know the risks they take, but they tell us they don't have any other choice," he added.

Vimard is also concerned that migrants who are picked up by the Libyan coast guard and returned to Libya are being condemned to a cycle of violence, trafficking and abuse. Last year, a CNN investigation exposed the migrant slave markets operating in Libya, and last month, the UN Refugee Agency warned that many people fleeing war and persecution are falling prey to criminal networks in the country.

"If Italy and the EU continue to place politics above people's lives, it will only lead to more loss of life," Vimard said.