As many as 240 people have died in two migrant shipwrecks off the Libyan coast Thursday, according to the United Nations Refugee Agency.

The deaths bring this year's toll to more than 4,220 migrants dead or missing in risky Mediterranean Sea crossings, the highest count on record.

The UN's Carlotta Sami said 31 survivors arrived on the southern Italian island of Lampedusa and reported the rubber dinghies they were travelling in had capsized in heavy seas shortly after leaving Libya.

Sami said 29 people survived the first wreck, while 120 people had gone missing. In a separate operation, two women found swimming at sea told rescuers another 120 people had died in their wreck.

In both cases, most people on board appeared to have been sub-Saharan Africans, but Sami said aid workers were still looking for details about the shipwrecks.

Aid workers in Malta prepare to search for migrants attempting to reach Europe by boat from the Libyan coast across the stretch of sea that has become the world's deadliest crossing point for migrants. (Juan Medina/Reuters)

Leonard Doyle of the International Organization for Migration said 12 bodies were recovered in one of the shipwrecks, 40 kilometres off the Libyan coast.

The UN said the increased number of deaths this year is partially due to the fact smugglers often use rubber dinghies, which are cheaper and easier to obtain. However, the boats are prone to deflation causing people to fall overboard and many migrants receive severe burns from exposure to fuel mixed with sea water collecting in the bottom of the dinghies.

Approximately 3,777 people died or went missing in the Mediterranean last year, the previous high.

Agencies rely heavily on survivor accounts to tally the number of people dead or missing in the dangerous journey across the Mediterranean in smugglers' boats.

Often, the boats sink in high seas making it difficult, if not impossible to recover bodies.