Havana ? Armed hijackers seized a ferry off Cuba’s coast Wednesday and threatened to toss passengers overboard if they could not go to the United States, setting off a negotiating drama on the high seas.

The FBI said the ferry was drifting in international waters about 60 miles off Key West and was sending hostage negotiators to the scene by helicopter to rendezvous with a U.S. Coast Guard cutter.

Fidel Castro’s government said it would handle the crisis in the Florida Straits. The U.S. Coast Guard confirmed that Cuba would take the lead role but said it was ready to assist.

“What we won’t do in any case is to use measures of force that put in danger the lives of the people aboard this boat,” Cuba said. It said 50 passengers were on board.

An FBI spokeswoman in Miami said later Wednesday that FBI agents at the scene were standing by on the U.S. Coast Guard cutter while Cuban authorities dealt with the situation.

The seizing of the vessel came a day after a Cuban passenger plane was hijacked to Key West, Fla., by a man who allegedly threatened to blow up the aircraft with two grenades that later turned out to be fake.

From a still earlier hijacking on March 19, six Cuban men are facing facing U.S. federal hijacking charges.

In a highly unusual move, the top U.S. diplomat in Havana on Wednesday night warned Cubans not to undertake any more hijacks, saying in a message read on communist-run television that they would be prosecuted and lose the right to seek American residency.

The message by James Cason, chief of the U.S. Interests Section, demonstrated growing worries about the possibility that such hijackings could end in violence or spark a migration crisis.

The latest crisis began early Wednesday when a group of people armed with three pistols and at least one knife hijacked the ferry, Havana said in a statement read on state television.

The ferry provides service between Havana and the small communities of Casablanca and Regla on the other side of Havana Bay.

There were no other details about what was transpiring between the hijackers and the Cuban and U.S. authorities out on the open sea.

The Cuban statement said two Cuban Coast Guard boats had followed the ferry out to the high seas, where they will remain to provide assistance in an emergency or to escort the ferry back to Cuba should the hijackers decide to turn the boat around.

Ferries like this one provide regular service in Havana, Cuba. Early Wednesday, a group of people armed with three pistols and at least one knife hijacked one of these ferries and threatened to throw some of the 50 passengers overboard unless allowed to sail to the United States, the Cuban government said.