It is feared that some 400 migrants drowned after their boat capsized 120 kilometres south of Lampedusa, the United Nations refugee agency said on Wednesday, urging more effective patrols of the Mediterranean.
Italian coastguard rescued 142 survivors from the shipwreck and recovered nine bodies, but survivors said there were over 500 people aboard the double-deck boat.
The United Nations High Commissioner for Refugees (UNHCR) said the death toll of around 400 was likely given the size of the ship.
"I was deeply shocked when hearing the news that another boat, an overcrowded boat capsized in the Mediterranean and where four hundred people died.
"This only demonstrates how important it is to have a robust rescue-at-sea mechanism in the central Mediterranean,” said UNHCR chief Antonio Guterres.
Almost 10,000 migrants have been rescued off southern Italy in recent days as warmer weather has prompted a surge in crossings from North Africa.
Italy's interior ministry has instructed officials throughout the country to be prepared to house the new arrivals, many of whom are children. And more boats are reported to be heading for the Italian coast.
An increasingly violent and chaotic situation in Libya has also contributed to the spike in the number of migrants attempting the perilous sea crossing.
Most of the people smuggling boats set sail from the virtually unpatrolled coast of Libya, which plunged into chaos after the 2011 ouster of longtime leader Muammar Gaddafi.
Libya's coastguard says it expects more attempted crossings later this week.
Italian coastguard are still carrying out most of the rescue operations in the Mediterranean as the European Union's more limited border security operation is only mandated to rescue boats within 30 miles of the shore.
The Italian government wound up a major maritime rescue operation in December on cost grounds and amid concerns that it was encouraging migrant crossings.
UN officials say well over 500 people have died in the Mediterranean since the start of the year, 30 times more than in the same period last year. That figure exclues the 400 people feared drowned on Monday.
Over 280,000 people entered the EU illegally last year, many fleeing conflict in Syria and Somalia and repression in Eritrea.
