A massive rescue operation is underway in the Mediterranean after a boat with hundreds of migrants capsized north of Libya, according to news reports.

The Italian coast guard said at least 49 people and 20 bodies had been pulled from the water, but as many as 700 may have been packed onto the rickety, 66-foot vessel, making the sinking the deadliest known migrant sea disaster in history, according to the Associated Press.

The coast guard said the vessel may have overturned when a Portuguese merchant vessel approached late Saturday and passengers simultaneously moved from one side of the ship to the other, unbalancing the vessel.

“They wanted to be rescued,” Barbara Molinario, a spokeswoman for the U.N. refugee agency in Rome, told the Guardian: “They saw another ship. They were trying to make themselves known to it.”

A coast guard official told the BBC that 20 ships and three helicopters were involved in the rescue effort.

Authorities told the AP that they have no immediate way of determining the exact number of passengers on the ship before it sank or how many might still be pulled from the water alive. Coast guard officials expect to arrive at a more precise figure after interviewing survivors.

In similar tragedies in recent years, the AP reported, the extreme depth of the sea has made the recovery of some bodies highly unlikely. The water in the area where the vessel sank is as much as three miles deep, according to the AP.

For now, officials said, the scene was chaotic, with large fuel stains, life jackets and wreckage from the boat floating in the ocean, according to the AP.

Maltese Prime Minister Joseph Muscat told the BBC that rescuers were “literally trying to find people alive among the dead floating in the water.”

This year, at least 900 migrants are thought to have died while crossing the Mediterranean, according to the BBC. The Guardian, however, estimates that the number may be as high as 1,500.

Last week, the AP reported, 400 migrants drowned when their boat capsized during a failed crossing.

“In the past week alone,” the BBC reported, “Italy’s coastguard rescued 10,000 migrants whose vessels ran into trouble.”

On Sunday, Pope Francis, an outspoken advocate for migrants, addressed the disaster by calling for tolerance, the Guardian reported. A day earlier, the BBC noted, he supported a call by Italy for the European Union to intervene in the growing migrant crisis on the Mediterranean.

“They are men and women like us — our brothers seeking a better life, starving, persecuted, wounded, exploited, victims of war,” he said Sunday from St. Peter’s Square.