An overcrowded fishing boat doubling up as a migrant boat in the Mediterranean capsized Wednesday about 15 miles north of Libya.

Rescuers managed to save 400 people, but 25 died when the boat initially over turned, and another 100 are assumed to have drowned, Italian officials said, according to the New York Times.

Another Coast Guard spokesman said that several boats were involved in the rescue effort, including an Irish naval vessel and a boat operated by Médecins Sans Frontières (Doctors Without Borders), which were the first to arrive at the scene.

While reports about the incident were sketchy, at least one of the boats reached the migrants' boat and was in the process of dropping rescue boats when it overturned.

Preliminary reports indicate that the fishing boat capsized when migrants moved to one side of the vessel after seeing the rescue boats approaching, according to The Wall Street Journal.

Estimates on the number of migrants on board were sketchy as well, as Italian authorities failed to verify the information given by the migrants in their initial mayday call Wednesday morning to the Coast Guard in Sicily.

Ireland's Minister for Defense Simon Coveney said that there was approximately 600 people aboard the capsized boat, but MSF spokeswoman Sibylle Berger said there could have been as many as 700, according to CNN.

Nongovernmental organizations have increasingly started helping in rescue operations in the Mediterranean under the umbrella of Triton, the joint rescue program run by the EU.

European leaders increased funds for search-and-rescue operations in April after over 700 migrants died when their boat capsized in waters between North Africa and southern Italy. Only 28 people survived the incident.

The International Organization for Migration, a human rights group, says that at least 2,000 people have died while trying to cross the Mediterranean since January, but it is not possible verify the exact death toll.