LONDON — A major search-and-rescue operation was underway Sunday after a migrant boat capsized in Libyan waters south of Italy. Most 700 thought to be on board were missing and feared dead.
Twenty-eight people have been rescued, while 24 bodies have been recovered, the Italian coast guard said on Twitter.
If confirmed, the disaster would be one of the worst seen during the decades-long migrant crisis in the southern Mediterranean, and would bring the total number of dead since the beginning of the year to more than 1,500, according to the United Nations Refugee Agency.
"A tragedy is unfolding in the Mediterranean, and if the EU and the world continue to close their eyes, it will be judged in the harshest terms as it was judged in the past when it closed its eyes to genocides when the comfortable did nothing," Maltese Prime Minister Joseph Muscat said.
UNHCR spokeswoman Carlotta Sami told NBC News that a survivor reported 700 people were on board the boat.
"We fear that hundreds may have died in the capsizing," Sami said.
But Italian Prime Minister Matteo Renzi said Italian authorities are "not in a position to confirm or verify" how many people were aboard. Eighteen ships, four helicopters and a plain had been diverted to help rescue efforts, Italian Prime Minister Matteo Renzi said during a news conference in Rome.
The Times of Malta reported that the incident occurred 120 miles south of Lampedusa.
The Italian official confirmed that navy vessels were involved in the operation, which was being coordinated by the Italian coast guard in Rome.
The boat is believed to have capsized when migrants moved to one side of the overcrowded vessel when a merchant ship approached.
"The first details came from one of the survivors who spoke English and who said that at least 700 people, if not more, were on board," Sami told SkyTG24 television.
"The boat capsized because people moved to one side when another boat approached that they hoped would rescue them."
Renzi said Europe was witnessing "systematic slaughter in the Mediterranean."
"How can we remain insensible when we're witnessing entire populations dying at a time when modern means of communications allow us to be aware of everything?" Renzi said at a political event in Mantua.
Renzi spoke by telephone to French President Francois Hollande and at a news conference later requested an emergency European Council meeting. "I can't believe that, faced with this tragedy, Europe doesn't feel the same spirit of solidarity it showed during other events," Renzi said.
"We need to stop human trafficking. We can't only save those people by rescuing them out at sea. We can save them also by not letting them leave [the coast], and be at the mercy of human traffickers," he said.
The latest capsize comes after more than 7,000 migrants — a record-breaking number — were rescued while trying reach Europe last weekend, according to the European Union.
The deadliest official death toll in the capsizing of a boat carrying migrants in the Mediterranean is 366 — the number of bodies recovered off the coast of Lampedusa on Oct. 3, 2013.
Amnesty International described the latest incident as a "man-made tragedy that could well have been avoided."
"What we are witnessing in the Mediterranean is a man-made tragedy of appalling proportions," John Dalhuisen, Amnesty International's Director for Europe and Central Asia, said in a statement. "These latest deaths at sea come as a shock, but not a surprise."
About 31,500 migrants have crossed the Mediterranean to Italy and Greece so far this year, according to the United Nations.
Reuters and The Associated Press contributed to this report. This article originally appeared on NBCNews.com.