Nearly 700 migrants are believed to have drowned when a packed boat capsized in international waters south of Italy's Sicily island on Saturday night, media reports said on Sunday.
A total of 28 migrants aboard the boat have been rescued and 24 bodies recovered, the Italian coast guard said on Sunday, according to Spanish news agency Efe.
Navy and coast guard ships continue searching for other survivors of the vessel that sailed from Libya and sank near Sicily.
After the boat carrying the migrants capsized, the Italian coast guard received a call for help and asked the Portuguese freighter King Jacob, which was transiting through the area, some 110 km off Libya, to provide assistance. It managed to recover 28 survivors, according to Italy's ANSA news agency.
The migrants "all shifted to the same side of the vessel and caused its sinking" as the freighter neared, the UN High Commissioner for Refugees (UNHCR) said.
"It might be one of the biggest tragedies to have happened in the Mediterranean," Carlotta Sami, spokesperson of the UNHCR, told Rai State television.
Thousands of migrants have arrived on Italy's shores in recent days, forcing the government to take emergency measures to deal with the crisis.
Pope Francis, meanwhile, called on the international community on Sunday to "act with decision and promptness" to prevent future tragedies in the Mediterranean.
The European Union's interior and foreign relations ministers plan to hold a joint meeting to discuss the migrant crisis in the Mediterranean, the European Commission said on Sunday.
European officials plan to discuss the formulation of a "wide-ranging" strategy to deal with the problem, the commission said.
The majority of the migrants taking to sea in an attempt to reach Europe are from North Africa and sub-Saharan Africa.
At least 218,000 migrants tried to cross the Mediterranean Sea last year to enter the European Union from its southern borders. Some 3,500 of them lost their lives in the attempt, the UNHCR said in a recent statement, adding that the trend is "expected to continue".