(CNN) More than 4,600 migrants are presumed to have died after attempting to cross the Mediterranean into Europe this year, the UN said Friday, following the latest deadly capsizings on the route.

An estimated 350 migrants are believed to have died in six incidents on the Mediterranean in just three days this week, the International Organization for Migration (IOM) said in a statement.

More than 130 people drowned from one boat that sunk Monday, with only 15 passengers surviving, while about a hundred people died or are missing on two other vessels on subsequent days. All three boats had departed from Libya and were traveling in rough seas and poor weather.

While the number of people attempting to crossing into Europe has dropped sharply this year -- down to 343,589 from 1.02 million in 2015 -- more people than ever are losing their lives on the journey.

The UN announced last month that 2016 was already a record year for deaths along the treacherous maritime route into Europe. The IOM -- the UN's migration agency -- says that 3,777 people died along the route last year -- while 4,621 are believed to have died so far this year.

"This means that 1,000 more deaths have been recorded this year than during the same period last year," the IOM said in a statement Friday.

Unsafe conditions

The IOM said that the six tragedies this week -- five of which occurred on the so-called Central Mediterranean route between Libya and Italy -- all occurred under similar circumstances.

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The crowded rubber dinghies began taking on water amid bad weather and choppy sea conditions, and that led to hundreds of deaths.

"What is really of great concern about these shipwrecks is that the bad weather did not stop the human smugglers from forcing people onto these unsafe dinghies," said Flavio Di Giacomo, spokesman for the IOM Coordination Office for the Mediterranean in Rome.

"From the first testimonies that we collected so far, it seems that the migrants were forced on board, even though they were clearly afraid of the rough seas. It was extremely dangerous in such bad weather. The rescues were also even more complicated under these conditions," he said.

Central route deadliest

The flow of migrants into Europe is roughly split between those taking the central route into Italy, and the eastern route from Turkey to Greece. But the vast majority of the deaths in the Mediterranean this year -- 4,139 -- have occurred on the treacherous central Mediterranean route.

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Libya is a busy departure point for illegal migrants seeking to reach Europe from North Africa, with well-established smuggling networks in the country capitalizing on the absence of an effective central government.

Much smaller numbers of migrants -- 5,445 -- take the western Mediterranean route to Spain, with 62 people dying attempting that journey so far this year.

An agreement in March between the European Union and Turkey has led to a huge drop in the number of migrants leaving Turkey for Greece, a much shorter and less perilous journey than the Libya-Italy route.