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.