MILAN — About 340 migrants have died or are missing after four Mediterranean Sea shipwrecks in the past three days, adding to the toll in the deadliest year on record for asylum-seekers risking the dangerous voyage to Europe, a migration organization said Thursday.

The shipwreck causalities bring to more than 4,500 the number of migrants who have died or disappeared crossing the Mediterranean this year, according to the International Organization for Migration figures. The total compares with the 3,770 people reported dead or missing last year, the previous record.

The group said Thursday the death toll is rising as smugglers force departures despite rough, winter seas.

‘‘What is shocking is the cruelty,’’ Flavio Di Giacomo, Italy spokesman for the organization, said. ‘‘The traffickers are forcing people to depart despite the prohibitive sea conditions. When they get to the beach, migrants who don’t want to go are forced to get on board, even with violence.’’

Di Giacomo said traffickers care little if the migrants make it alive. ‘‘Once you pay, you can’t go back,’’ he said.

The count from the recent shipwrecks was based partly on the rescue overnight by Doctors without Borders of 27 migrants, who reported that more than 130 people had been on board their rubber dinghy when it sank, Di Giacomo said. Seven bodies have been recovered.

In another incident, 15 survivors rescued by a mercantile ship about 30 miles from Libyan shores reported that some 135 people died when their smugglers’ boat capsized. Five bodies were recovered.

Another ship rescued 23 migrants, who reported that more than 120 people had been on board when they sank overnight Tuesday. Six bodies were recovered Tuesday in the fourth rescue, of 114 people.

The impossibility of recovering bodies of migrants lost at sea means that humanitarian organizations must rely on the accounts of survivors to tally the number believed drowned.

So far this year, nearly 160,000 migrants have arrived in Italy, up 13 percent from last year.

Associated Press