By RENATO PEREZ

  The case of the hijacked Cuban ferryboat took a turn Thursday when, according to authorities at the port of Mariel, the hijackers refused to surrender and threatened to harm their hostages if they were not given fuel for another try at traveling to Florida.

  Shortly before noon Thursday, President Fidel Castro himself arrived at Mariel, about 30 miles west of Havana, presumably to oversee the situation.

  An official statement read on television and radio at 1 p.m. stated the government's hard-line reaction to the hijackers' demands.

  ''Force will be used if the hostage situation becomes critical,'' said the statement, which was reproduced in the Internet sites of the official newspaper Granma and the state-run news agency AIN.

  The hijackers have agreed to release three of their approximately 50 hostages, the authorities said.

  The ferry, a flat-bottomed boat that normally carries commuters across Havana Bay, between Old Havana and the eastern suburbs of Regla and Casablanca, was hijacked early Wednesday by eight or 10 armed men who ordered the captain to take them ``to Miami.''

  The boat ran out of fuel about 30 miles off the coast of Havana and eventually the hijackers allowed Cuban Frontier Guard vessels to tow it back to Cuba.

  However, the Cuban statement said, the hijackers ''held knives to the necks of women hostages when they arrived at 10:12 p.m. [Wednesday] to a dock at the port'' of Mariel.

  The boat is tied to the dock with a 10-yard rope, the statement said.

  Authorities blocked access to international journalists, but several dozen officials, a fire truck and numerous government vehicles could be seen near the boat from a hill high above the bay.

  All through the morning, the authorities ''tried to persuade [the hijackers] to free the hostages,'' and ``after repeated pleas, they only agreed to release three: two women and one man in a delicate state of health.

  ''They responded to all proposals with a demand for fuel,'' the statement said.

  The authorities described the men as ``persons with awful police records, ferocious behavior and very low levels of culture.''

  ''They are known to have a Makarov pistol and numerous knives,'' the note said. The Makarov is a Russian-made 9mm pistol. There was no mention of two other firearms -- ''.38-caliber weapons'' -- the hijackers allegedly had, according to a government statement on Wednesday.

  The roads leading to Mariel and the port itself were heavily guarded by police and other security personnel in early afternoon Thursday.

  Herald wire services were used to prepare this report.