WASHINGTON — The Bush administration today ruled out any immediate change in policy toward Cuba despite Fidel Castro’s resignation as president, deriding his brother and heir apparent, Raul, as “dictator-lite.”

Administration officials led by President Bush expressed hope that Mr. Castro’s decision to step down would usher in a period of democratic transition on the communist-run island, but stressed they doubted that would happen under Raul Castro and said it was unlikely the nearly 50-year-old American economic embargo on Cuba would be lifted.

“I can’t imagine that happening any time soon,” Deputy Secretary of State John Negroponte said when asked if Washington would lift the embargo, which has been the centerpiece of American policy toward Cuba since it was first imposed in 1960 and strengthened in 1962.

The ailing Mr. Castro, 81, who has called the trade embargo “criminal” and claims its impact has run into the tens of billions of dollars, announced earlier today he would not accept another term in office when parliament meets to elect a new president this weekend. Despite constant American criticism and sanctions, Mr. Castro outlasted nine American presidents.

In Rwanda, Mr. Bush said he hoped the end of Mr. Castro’s presidency will launch a transition to democracy for the Cuban people.

“They’re the ones who suffered under Fidel Castro,” he told a news conference. “They’re the ones who were put in prison because of their beliefs. They’re the ones who have been denied their right to live in a free society. So I view this as a period of transition and it should be the beginning of the democratic transition in Cuba.”

The State Department offered similar sentiments, but stressed it was not optimistic for any kind of quick change under Raul Castro, to whom Fidel Castro ceded power temporarily in July 2006.

“The changing of the guard is not significant of and by itself,” a spokesman, Tom Casey, told reporters, repeatedly referring to Raul Castro as “dictator-lite” or “Fidel lite.”

“It will be significant if in fact it leads to greater openness and freedom for the Cuban people and ultimately to a democratic transition,” he said. But, he cautioned that “the general analysis is that Raul Castro is ‘Fidel lite’.”

“He is simply a continuation of the Castro regime, of the dictatorship,” Mr. Casey said, adding: “There are some very clear indications out there that what this transition would potentially become, or at least what Fidel and his minions would like it to become, is a transfer of authority and power from one dictator to dictator-lite, from Fidel to Raul.”

Mr. Casey said America stood ready, however, to assist the Cuban people in a true transition to democracy.

Rep. Ileana Ros-Lehtinen, a Miami-area Republican who was born in Havana, said Mr. Castro’s resignation was irrelevant because his regime had already “done great harm to the suffering Cuban people.”

“It matters nothing at all whether Fidel, Raul or any other thug is named head of anything in Cuba,” she said. “What the people want is freedom to vote in multiparty elections that are internationally supervised and freedom to express their dissent from the oppressive regime. The Communist machinery is enslaving them so it doesn’t matter who the thug of the moment will be.”

Senator Menendez, Democrat of New Jersey, who is of Cuban descent, said Mr. Castro’s resignation “is not the cause for celebration that some would believe.”

“This does not represent the replacement of totalitarianism with democracy. Instead, it is the replacement of one dictator with another,” he said in a statement.

The Coast Guard, meanwhile, has not added any additional patrols in light of Mr. Castro’s resignation, a Coast Guard spokesman, Chris O’Neil, said.

Mr. O’Neil, speaking from Florida, said there have been no indications or warnings of a mass migration. “The threat has not changed,” Mr. O’Neil said.

Mr. Bush said he anticipates debate about Cuba’s future, and that some people will say: “Let’s promote stability.”

“In the meantime, political prisoners will rot in prison and the human condition will remain pathetic in many cases,” he said.

Mr. Bush noted that he had met with the families of some of prisoners, and that their release should be the first step of any transition to democracy.

“It just breaks your heart to realize that people have been thrown in prisons because they dare speak out,” he said.

While Mr. Bush expressed hope for democratic change, Mr. Castro’s decision appeared to position his brother, Raul Castro, to succeed him as president.

“The international community should work with the Cuban people to begin to build institutions that are necessary for democracy,” Mr. Bush said.

“Eventually, this transition ought to lead to free and fair elections — and I mean free, and I mean fair — not these kind of staged elections that the Castro brothers try to foist off as true democracy,” Mr. Bush said.

“The United States will help the people of Cuba realize the blessings of liberty,” Mr. Bush said.