At least 20 irregular asylum seekers died after a boat overturned in the Mediterranean on Wednesday, with many more still missing, the UN's refugee agency (UNHCR) said.

"Deep anguish for the umpteenth shipwreck off the coast of Lampedusa, where UNHCR is now assisting the survivors. It looks to be 20 bodies found and as many missing," wrote the agency's spokesperson, Filippo Ungaro, on social media.

Italian news agency Radio Radicale said the boat had been carrying 97 people when it shipwrecked 14 miles southwest of Lampedusa.

Details remained sketchy but Save the Children Italy said that a baby girl, aged one-and-a-half, appeared to be lost in the shipwreck.

RaiRadio1 reported between 12 and 17 irregular refugees missing, and said that 60 survivors had been transported to safety on the island.

The boat, which had already overturned, was spotted from the air by a plane from Italy's financial police, it said.

People heading to Italy from North Africa often cross in leaky or overcrowded boats via the central Mediterranean route, one of the world's deadliest, and arrive in Lampedusa.

The UNHCR said on Wednesday there have been 675 deaths on the central Mediterranean route so far this year.

As of Wednesday, 38,263 irregular refugees have arrived on Italy's shores this year, according to the Interior Ministry.