Dozens of passengers in a crowded rubber boat fell into the sea and may have drowned as a rescue vessel neared, survivors reported as they arrived in Italy to join thousands of other migrants rescued in the Mediterranean in recent days.

Giovanna di Benedetto of the aid group Save the Children said the exact toll is not known, but she said survivors, in their first interviews with aid officials, reported that many people fell into the sea and could not swim after their boat either deflated or got into difficulty.

The incident is believed to have occurred on Sunday in the sea between Libya and Sicily when a commercial vessel, the Zeran, approached the rubber boat to rescue the migrants. The survivors were brought to Catania.

The weekend saw a dramatic increase in rescues as smugglers in Libya took advantage of calm seas and warm weather to send thousands of would-be refugees out into the Mediterranean in overloaded rubber boats and fishing vessels. The coastguard reported that nearly 7,000 people were rescued in the three days ending Sunday.

The latest deaths come on top of the estimated 800 migrants who are believed to have drowned last month when their boat capsized off Libya with hundreds of passengers locked in the hold by smugglers. A few days earlier, a further 400 people were feared drowned in another capsizing.

After the deaths, the EU held an emergency summit and agreed to contribute more boats and patrol aircraft to Mediterranean rescue efforts.

Even with the increased EU response, commercial cargo ships are increasingly being called on by Italy's coast guard to respond to migrants in need, as required by the law of the sea.

Catania prosecutor Giovanni Salvi complained last month that these commercial crews are often not trained or equipped to conduct rescues and that lives can be lost when migrants suddenly shift places on their unseaworthy boats as they try to get off.

Mr Salvi later backtracked and praised the work and commitment of the commercial vessel King Jacob, which had come to the aid of the boat in which the 800 passengers were trapped in the hull and capsized during the rescue.

In addition to commercial vessels, aid groups are pitching in: The Phoenix, a 130ft refitted yacht, arrived in Pozzallo, Sicily, today with 369 mostly Eritrean migrants who were rescued by the crew of the Migrant Offshore Aid Station (MOAS).

MOAS was founded in 2013 by a Maltese-based American-Italian family and now works with Italian search and rescue authorities to locate and provide first aid to migrants in need. The 20-member crew includes a team from Doctors Without Borders.

The arrivals are stretching Italy's already overstretched migrant reception centres, with new arrivals being sent inland to be screened for asylum or in many cases, to continue on their journeys north unofficially.

"We are about to reach the limit of our capacity to accommodate them," said the Rev Vincenzo Federico, director of the Caritas Catholic aid group in Salerno, where 652 migrants from Ghana, Nigeria and Gambia arrived aboard the Italian navy ship Bettica.

Volunteers and medical personnel at the port donned medical garb to welcome the migrants because many are suffering from scabies. MOAS reported it also treated some migrants for injuries suffered during beatings and attacks - a reference to the violent treatment the migrants suffer in Libya at the hands of their smugglers.