At least 30 migrants are missing following two shipwrecks off the Italian island of Lampedusa, according to survivor testimony, as rescuers Sunday winched to safety others trapped on the rocks by rough seas.

Around 28 people were reported lost at sea by survivors on one boat, while three were reported missing from the second, after both went down in stormy weather on Saturday, said the International Organization for Migration (IOM).

Both were rickety iron boats believed to have set off from Sfax in Tunisia on Thursday.

Italy's coastguard said it had saved 57 survivors from the two shipwrecks, and recovered the body of a woman and a minor.

It released dramatic footage Sunday of the rescues, in which people could be seen carried high on the crests of vast waves, while a coastguard vessel soared and plunged nearby.

While some people tried to climb onto the vessel as it rocked, others, wearing black rubber rings, clung desperately to one another in a human chain.

Cultural mediators with the IOM believed there were "at least 30 people missing" after speaking to those pulled from the waves, press officer Flavio Di Giacomo told AFP.

- 'Criminal lunatic' -

An investigation into the shipwrecks has been opened in Agrigento, on the nearby Italian island of Sicily.

Agrigento's chief of police Emanuele Ricifari said the traffickers would have known bad weather was forecast.

"Whoever allowed them, or forced them, to leave with this sea is an unscrupulous criminal lunatic," he told Italian media.

"Rough seas are forecast for the next few days. Let's hope they stop. It's sending them to slaughter with this sea," he said.

As the stormy weather continued, an alpine rescue team and the air force were Sunday lifting to safety migrants trapped on a rocky part of Lampedusa's coastline.

The Sicilian Alpine Rescue (CNSAS) said there were more migrants on the rocks than the 20 initially reported -- perhaps as many as 40.

The migrants have been there since late Friday, after their boat was tossed onto the rocks by strong winds.

They have been provided with food, water, clothes and emergency thermal blankets by the Red Cross, but the coastguard had been unable to rescue them by sea due to the high waves.

The first six people to winched up into a helicopter and taken to the island's airport were women, CNSAS said.

- Iron boats -

The Central Mediterranean migrant crossing from North Africa to Europe is the world's deadliest.

Over 1,800 people have died attempting it so far this year, Di Giacomo said -- almost 900 more than last year.

"The truth is that figure is likely to be much higher. Lots of bodies are being found at sea, suggesting there are many shipwrecks we never hear about," he said.

The number of bodies found has increased in particular on the so-called Tunisian route, which has become increasingly dangerous, Di Flavio said, because of the type of boats used.

Sub-Saharan migrants are being put out to sea by traffickers "in iron boats which cost less than the usual wooden ones, but are utterly unseaworthy, they easily break up and sink", he said.

Migrants also often have the engines stolen from their boats at sea, so that traffickers can re-use them.

Nearly 92,000 people have landed on Italy's shores so far this year, according to the interior ministry, over twice the number in the same period last year.