Cuba executed three men yesterday convicted of terrorism for hijacking a passenger ferry last week in an abortive attempt to flee to the United States, the Cuban government reported.

The men, Lorenzo Enrique Copello Castillo, Barbaro Leodan Sevilla Garcia and Jorge Luis Martinez Isaac, had been given three days to appeal their conviction on Tuesday for "very grave acts of terrorism," according to a Cuban government statement, released in Havana.

The death sentences, upheld on appeal by Cuba's Supreme Tribunal and by the ruling Council of State, were carried out at dawn, the statement said. The executions were reportedly carried out by firing squad.

The United States, which cooperates with the government of Cuban President Fidel Castro on hijackings, responded cautiously to news of the executions. A State Department spokesman said the trials of the men, along with four others who received life sentences, "may have been a result of summary proceedings."

"The United States condemns hijacking which is an act of terrorism," said Lou Fintor, a State Department spokesman. At the same time, he said the Bush administration was concerned about due process. "Summary proceedings are a hallmark of totalitarian dictatorships like Cuba. Due process allows an appropriate judicial process to carefully identify and punish serious crimes like hijacking and guard against manufactured charges."

The U.S. response to the case was complicated by international agreements observed by both governments for the return of hijackers. A day before the ferry hijacking, a Cuban passenger plane was hijacked to Key West, Fla., and another Cuban plane was hijacked two weeks earlier. The hijackers were imprisoned, while some passengers opted to remain in the United States.

Cuba said yesterday it foiled a fourth hijack attempt when four armed men were arrested outside the airport on the Isle of Youth, 85 miles south of Havana.

The series of hijackings coincided with a crackdown on dozens of dissidents in Cuba and rising tensions with the United States.

The ferry was seized early in the day on April 2, when at least six men commandeered the vessel in Havana Bay and ordered the captain to sail to the United States.

Two Cuban Coast Guard vessels chased the 45-foot ferry, the Baragua, to the Straits of Florida, in international waters, where it ran out of fuel. The hijackers allegedly threatened to throw some of the 50 passengers overboard before they were persuaded to allow the boat to be towed back to Cuba for refueling.

The ferry was towed 30 miles to Mariel, west of Havana, and the following day, security forces stormed the boat and seized the hijackers. According to one account of the seizure, a French hostage jumped into the water, confusing the captors, who were then seized by police. The ferry passengers were said to be unharmed.

"These executions are an assassination draped in legalities," Elizardo Sanchez, head of the anti-government Cuban Commission for Human Rights, said in Havana, according to Reuters.

The executions followed a government crackdown on dissent that has been widely condemned by human rights organizations, the Bush administration and by supporters of improved U.S. ties with Cuba. About 80 human rights activists have been arrested, and a number of them have been sentenced to long prison sentences.

On Thursday, Secretary of State Colin L. Powell criticized Cuba for carrying out "the most significant act of political repression in decades."

"We call on Castro to end this despicable repression and free these prisoners of conscience."

The return to hard-line tactics against domestic dissent repression has surprised and angered advocates of easing the decades-old U.S. sanctions against Cuba.

Sen. Christopher J. Dodd (D-Conn.), one of the strongest congressional supporters of engagement with Cuba, denounced the crackdown. The arrest and lengthy detentions of dissidents "call into question the very legitimacy of the Cuban state," he said, describing government actions as "reprehensible" and "tyranny."

Wayne S. Smith, a senior fellow at the Center for International Policy, and a frequent critic of U.S. policy toward Cuba, dismissed the idea that Castro timed the crackdown while U.S. attention was focused on the war in Iraq.

"They knew we would notice and we would react very strongly and we have," said Smith, who served as chief of the U.S. Interests Section in Cuba from 1979 to 1982.

Cuban officials have condemned recent high-profile activities of James Cason, the current head of the U.S. Interests Section in Havana, who has held frequent public meetings with Cuban dissidents. The Cuban government says the dissidents are supported by the United States and Miami-based anti-Castro exiles.

Staff writer Karen DeYoung contributed to this report.

Cuban police stand near the home of convicted ferry hijacker Barbaro Leodan Sevilla, who was executed along with two accomplices.