ROME, Italy -- At least 30 migrants are missing following two shipwrecks off the Italian island of Lampedusa, according to survivor testimony, the UN's migration agency said Sunday.

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.

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

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 rough seas were 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.

- Trapped -

As the stormy weather continued, firebrigade and alpine rescue teams were preparing Sunday to pull to safety some 20 migrants trapped on a rocky part of Lampedusa's coastline.

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 has been unable to rescue them by sea due to the high waves.

Should the winds not drop, rescuers will begin winching them up the 140-metre (460 foot) high cliff to safety, media reports said.

Should the winds not drop, rescuers will begin winching them up the 140-metre (460 foot) high cliff to safety, media reports said.

The Central Mediterranean 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.