Italian rescuers have found 18 bodies on a boat carrying hundreds of asylum-seekers and another died while being evacuated, the ANSA news agency reports.
All were apparently killed by toxic fumes from the boat's engine.
The boat, with 600 people on board, was intercepted on Saturday south of the island of Lampedusa, Italy's southernmost point and a major landing point for migrant Mediterranean crossings from Libya as it is closer to Africa than mainland Italy.
Two more people have been evacuated by helicopter from the boat and been taken to Palermo hospital in Sicily in a serious condition, the report said.
The coastguard and navy could not be reached for comment.
ANSA said Italian authorities were first alerted by a merchant ship in the area, roughly 80 nautical miles (148km) from Lampedusa.
Meanwhile, a merchant vessel, the Panamanian-flagged City of Sidon, arrived in Porto Empedocle port in Sicily on Saturday with 61 migrants on board - the survivors of another shipwreck close to Libyan waters.
Their boat was intercepted on Thursday, 36 nautical miles north of Tripoli and it sank as they were being rescued with 102 people on board, meaning that 41 are feared dead, ANSA reported.
The survivors were from Gambia, Ghana and Mali, it said.
Italian media on Saturday also said a navy warship arrived in the port of Salerno with 2,186 migrants rescued in recent days, adding that they came from Egypt, Eritrea, Ghana, Somalia and Syria.
There has been a sharp rise in migrant landings in recent weeks because of the calm summer weather and growing lawlessness in Libya, with hundreds of migrants now being intercepted by Italian authorities every day.
About 80,000 migrants are now believed to have landed in Italy so far this year, higher than the previous record of some 60,000 arriving in 2011.
Most of them come from Eritrea, Somalia and Syria, but there are also asylum seekers arriving from Afghanistan, Pakistan and other parts of Asia as well as sub-Saharan Africa.
Thousands have died in the crossings, either by drowning, dehydration or being suffocated by toxic fumes.
Earlier this month, Italian officials discovered the bodies of 45 migrants in the hold of a fishing boat, where they appeared to have suffocated after being locked in by traffickers.