Rome (AFP) – As many as 700 migrants were feared drowned Sunday after their packed boat capsized off Libya in what was described as the deadliest such disaster to date in the Mediterranean.

The UN refugee agency (UNHCR) and Italy’s coastguard said only 28 people had survived the wreck. Their testimonies suggested there had been about 700 people on board the 20-metre (70-foot) fishing boat, officials said.

“It seems we are looking at the worst massacre ever seen in the Mediterranean,” UNHCR spokeswoman Carlotta Sami said.

The Maltese navy said it had received an alert about the stricken vessel around midnight (2200 GMT) on Saturday.

The vessel was about 70 miles (126 km) off the Libyan coast and 110 miles (177 km) south of the Italian island of Lampedusa when it issued a distress call, Italy’s coastguard said.

The coastguard instructed a nearby merchant ship to go its aid and it was when the Portuguese vessel arrived at the scene that the fishing boat capsized, most likely as a result of the terrified passengers stampeding to one side in their desperation to get off, the UNHCR’s Sami said.

A total of 17 boats were in the area on Sunday morning, engaged in what appeared to be an increasingly fruitless search for survivors. Italy’s coastguard said 24 bodies had been recovered from the waters around the site of the accident.

The tragedy is the latest in a growing catalogue of mass drowning of migrants attempting to reach the European Union on overcrowded, unseaworthy boats run by people smugglers who are able to operate out of Libya with impunity because of the chaos engulfing the North African state.

The most serious incident prior to Sunday occurred off Malta in September 2014. An estimated 500 migrants drowned in a shipwreck caused by traffickers deliberately ramming the boat in an attempt to force the people on board onto another, smaller vessel.

In October 2013, more than 360 Africans perished when the tiny fishing boat they were crammed onto caught fire within sight of the coast of Lampedusa.