About 340 migrants have died or gone missing in four Mediterranean Sea shipwrecks over the past two and a half days, a migration organisation says.

Flavio Di Giacomo, Italy spokesman for the International Organisation for Migration, said that brings to over 4,500 the number of migrants who have died or disappeared so far this year making the risky sea voyage, the deadliest year on record.

He said the recent toll includes a 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. Seven bodies were recovered.

Mr Di Giacomo said the toll is increasing this year as smugglers are forcing departures despite rough winter seas.

"What is shocking is the cruelty," Mr Di Giacomo 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."

He said traffickers care little whether the migrants make it alive. "Once you pay, you can't go back," he said.

The International Organisation for Migration said the total compares with the 3,770 people reported dead or missing last year, the previous record.

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 on Tuesday. Six bodies were recovered in the fourth rescue, of 114 people.

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

Tens of thousands fleeing war, poverty and persecution seek to reach safety in Europe in smugglers' boats.

The European Union's Frontex border agency said a record 27,500 migrants were rescued and brought to Italy in October, the highest monthly total ever in the central Mediterranean and twice as many as in previous months.

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