LONDON and ATHENS -- Rescue efforts were underway Wednesday after a fishing vessel carrying migrants capsized and sank in the Mediterranean Sea, about 54 miles off the coast of Pylos, Greece.
At least 104 people have been rescued and 78 have died, the Greek Coast Guard said. Unconfirmed early estimates put hundreds of migrants -- perhaps as many as 650 -- on board when the boat began to sink at about 2:30 a.m. local time.
None of the rescued was wearing a life jacket, the coast guard said.
The Greek Coast Guard was in charge of the rescue operation, which included six Coast Guard vessels, a Greek Navy frigate, a military transport plane, an Air Force helicopter, several private vessels and a drone from the European Union border protection agency.
Survivors are being brought to a hospital on Kalamata, a city in southern Greece, officials said. Mayan Queen IV, a 300-foot yacht, picked up some survivors.
The Coast Guard had contacted the vessel midday on Tuesday as it traveled through international waters, according to a statement released Wednesday afternoon.
A ship approached the fishing boat with supplies later on Tuesday, but passengers refused further help, saying they wanted to continue on their path toward Italy, the Coast Guard said.
The number of illegal border crossings detected in the Central Mediterranean nearly quadrupled in the first four months of the year from the same year-earlier period, Frontex, the European Border and Coast Guard Agency, said last month.
More than 42,200 such crossings were recorded from January through April, Frontex said. That amounted to the highest level recorded since Frontex began collecting data in 2009, the agency said.
"Organised crime groups are taking advantage of political volatility in some departure countries to increase the number of migrants they smuggle across EU borders," Frontex said in a statement in May.
Wednesday's capsizing was another "tragedy in the Aegean," the U.N.'s International Organization for Migration said on Twitter.
The group called for comprehensive action including more pathways for legal migration.
ABC News' Joe Simonetti contributed to this report.