A rubber boat packed with 147 migrants sank in the Mediterranean and all but one of its passengers drowned, the sole survivor - a 16-year-old Gambian boy - told rescuers, the International Migration Organization (IOM) said on Wednesday.

A Spanish frigate, the Canarias, found the boy hanging onto a piece of debris in the sea on Tuesday. He was transferred to an Italian Coast Guard ship and brought to the Sicilian island of Lampedusa early on Wednesday.

"He was very tired when they found him. He's resting now, so we'll have more details later," said IOM spokesman Flavio Di Giacomo in Rome, after speaking to staff in Lampedusa.

"The boy said they left Sabratha, Libya, a couple days ago on a rubber boat with 147 sub-Saharan Africans on board, including five children and some pregnant women," Di Giacomo said.

In the past two days, rescuers have picked up more than 1,100 migrants at sea, and recovered one body, Italy's Coast Guard said. The Coast Guard did not comment on the latest shipwreck.

So far this year nearly 600 migrants have died trying to reach Italy from North Africa, IOM estimates, after some 4,600 deaths last year. Migrant arrivals to Italy are up more than 50 percent this year on the same period of last year.

Early on Wednesday the Golfo Azzurro, a humanitarian vessel, rescued about 400 migrants - mainly from Morocco, Algeria, Libya, Gambia and Bangladesh - including 16 women and two children.

They were found drifting in a wooden boat without power about 16 kilometres (10 miles) off the coast of Sabratha, the most frequently used departure point currently used by people smugglers in Libya, and will now be transported to Sicily.

"The migrants kissed and hugged their rescuers and sang songs" after they were brought to safety, said Reuters photographer Yannis Behrakis, who is onboard the Golfo Azzurro.

"My brother back home convinced me to make the trip," said Gambian Kalifa Kujabi, 17, after the rescue. He said he played for Gambia's soccer academy and paid $600 for the passage. "My brother said that I can only have a future as a soccer player in Europe."

Italy's parliament on Wednesday approved a law aimed at protecting unaccompanied minors in a measure welcomed by humanitarian groups.

Also on Wednesday, the Senate voted to pass a decree that foresees new detention centres for migrants who are to be deported, cutting the length of the appeals process for those whose asylum requests have been rejected. The decree now goes before the lower house.