A boat carrying nearly 100 migrants has capsized off the Italian island of Lampedusa, killing at least 20 people and leaving another dozen missing, the United Nations refugee agency says.

Sixty survivors have been brought to a centre in Lampedusa, UNHCR spokesman Filippo Ungaro said.

According to survivor accounts, there were 92 to 97 migrants on board when the boat departed Libya.

Authorities have recovered 20 bodies and were searching for another 12 to 17 survivors, according to the UNHCR.

Know the news with the 7NEWS app: Download today

It was not immediately known how long the migrants had been at sea.

According to the UNHCR, 675 migrants have died making the perilous central Mediterranean crossing so far this year, not counting the latest sinking.