BY NANCY SAN MARTIN

  Signs of internal discontent in Cuba increased Wednesday and rumors of a brewing mass exodus spread as alleged armed men staged the third hijacking in two weeks in an effort to reach Florida.

  In addition, Cuban prosecutors said they will seek long jail terms for political opponents -- an apparent effort to stifle dissent.

  Underscoring the U.S. government's alarm that the incidents could lead to a migration crisis, America's highest-ranking diplomat in Havana issued a rare statement via Cuban television Wednesday night warning that those who hijack planes or boats would be prosecuted and lose the right to seek residency in the United States.

  The unusual move by James Cason, chief of the U.S. Interests Section, followed unprecedented action by Fidel Castro on Tuesday when he personally became involved in trying to dissuade a man from commandeering a plane that ultimately landed in Key West.

  ''Such acts are extremely serious violations of international law and of U.S. law,'' Cason said of the hijackings in his message, read by an announcer in Spanish on the regular nightly broadcast. ``Any individual of any nationality, including Cuban, who hijacks an aircraft or vessel to the United States will be prosecuted with the full force of the U.S. legal system.

  ``The individual convicted of such offenses can expect to serve lengthy sentences in federal penitentiaries. Once convicted of such an offense the individual, including a Cuban, would be rendered permanently ineligible for lawful permanent residence in the U.S.''

  No top U.S. diplomat in recent memory has asked the Cuban media to broadcast and publish a message. It was expected to be printed today in Granma, the Communist Party daily.

  The hijacked ferry, carrying as many as 50 people, became the focus of an hours-long international standoff as U.S. and Cuban authorities argued over who would take charge of defusing yet another apparently illegal act involving both countries. Cuban authorities ultimately took charge and it appeared the ferry was headed back to Cuban waters.

  Cason's televised message came as human rights activists announced that at least 10 jailed dissidents will face life imprisonment in trials set to begin today.

  The harsh action against the dissidents, coupled with the hijackings, ''suggest a pattern of cracks in the wall,'' experts say, with some saying this could expand to a
  full-blown crisis reminiscent of mass exoduses in 1980 and 1994.

  ''This is the same pattern that was happening prior to Mariel with people trying to leave and bickering between the United States and Cuba,'' said Jaime Suchlicki, director of Institute for Cuba and Cuban American Studies at the University of Miami.

  ``All this may be a distraction set up by the Cuban government or it may be a prelude to a migration crisis with people trying to flee either through Guantánamo or
  directly to South Florida.''

  The personal involvement of Castro in a failed attempt to stop an airplane hijacking on Tuesday added to the impression that the issue has become a matter of urgent concern.

  ''Castro doesn't really want relations with the United States to deteriorate any further,'' said Antonio Jorge, an international relations professor at Florida International University. ``He has enough trouble . . . with this repressive wave against dissidents.''

  The arrests may have contributed to recent illegal departures, some analysts said, because they underlined the sense of fear already burdening Cubans who face a
  worsening economic slump.

  ''It's a bad ambience in Cuba right now,'' said Lisandro Pérez, director of the Cuban Research Institute at FIU. ``What all this means is that things are going to get worse before it gets better.

  'There are a lot of parallels with what happened in 1994 and 1980. History says that if the Cuban government gets fed up, it'll say, `You want to go? Go.' ''

  State Department officials said they would monitor developments and try to avert a migration crisis. Officials said they would continue to condemn the arrests of dissidents and rally international pressure to obtain their release.

  Cuban prosecutors are seeking life sentences for at least 10 of as many as 83 dissidents who have been detained.

  Among them are opposition political leaders Osvaldo Alfonso Valdés and Héctor Palacios as well as Martha Beatriz Roque, an economist and vocal government critic. Other prominent government opponents such as Raúl Rivero, the best known of Cuba's independent journalists, could get as much as 20 years behind bars, according to sources who have been monitoring the cases.

  The four, along with dozens of others, have been accused of working with U.S. diplomats in Havana to subvert the Castro government and being mercenaries on the U.S. payroll.

  The anticipated speedy trials are scheduled to begin today in at least four different Havana courthouses.

  Ricardo Alarcón, president of Cuba's National Assembly, said this week that authorities had sufficient evidence to try the dissidents, adding that most nations had laws ``to defend their sovereignty.''

  Many of the dissidents will likely be charged with violating a much-criticized law that makes it a crime to support subversion and acts that would break the internal order and destabilize the country.

  Even as Wednesday's hijacking unfolded, Cuba issued a notice in which the hijackers claimed to be armed and threatened to throw people overboard if they were not allowed to go to the United States. The statement blamed U.S. policy.

  ''Notice the extent of blackmail utilized by the terrorists and delinquents, stimulated by the policy of tolerance, resource, benefits, privileges, shelter and propaganda the governments of the United States have applied for more than 40 years in their effort to create and support counter-revolutionary movements to destroy the Cuban Revolution,'' the statement said.

This report was supplemented with material from The Associated Press.