WASHINGTON (CNN) -- The U.S. embargo on Cuba will remain in place despite Fidel Castro's announcement that he's resigning as Cuba's leader, Deputy Secretary of State John Negroponte said Tuesday.
Condoleezza Rice chairs a 2005 meeting of the U.S. group working on plans for a post-Castro Cuba.
Asked whether Castro's resignation would change U.S. policy, Negroponte said, "I can't imagine that happening any time soon."
President Bush said the move should spark "a democratic transition" for the communist island nation.
"The international community should work with the Cuban people to begin to build institutions that are necessary for democracy and eventually this transition ought to lead to free and fair elections," Bush said at a news conference in Rwanda during his five-nation tour of Africa.
"I believe that the change from Fidel Castro ought to begin ... a democratic transition," he said. "The United States will help the people of Cuba realize the blessings of liberty." Watch Bush react to Castro's decision »
Castro said he was stepping down Tuesday as president of Cuba and commander in chief of its military, according to a letter published in the country's state-run newspaper, Granma.
The United States and Cuba, which have no formal diplomatic relations, have been at odds for decades, but tensions between the two countries have increased in the past two years.
The Bush administration has tightened the four-decades-old U.S. embargo on the island, increased Radio Marti news broadcasts into Cuba, curtailed visits home by Cuban-Americans and limited the amount of money Cuban-Americans can send to relatives. Watch why the U.S. plans no change in policy toward Cuba »
The United States also has been working on plans for a post-Castro Cuba.
A 2006 report by the Commission for Assistance to a Free Cuba laid out the framework for Washington's possible response in the event of Castro's incapacitation or death.
The U.S. response could include tens of millions of dollars in humanitarian and economic aid but might also be dependent on a transitional government that's committed to democracy.
Castro transferred powers to his brother Raśl after receiving treatment for intestinal problems in 2006. Raśl Castro is considered more pragmatic than his older brother but hasn't shown any inclination to invite the United States to launch a full-scale democratic push and overhaul of the country's institutions.
At least one activist in Miami, Florida, said Castro's resignation does not mean Cuba is any closer to democracy.
"It doesn't mean any change to the system. It doesn't mean there will be freedom for the Cubans. One big dictator is replacing the other," said Janisset Rivero, executive director of Cuban Democratic Directorate, which works with dissidents in Cuba.
"It will be a big deal when political prisoners are released, when political parties are allowed to organize, when the country stops being ruled by a single party," Rivero said Tuesday. Watch what Castro's resignation means for Cuba »
Sen. Mel Martinez, R-Florida, also said: "Today is not the ultimate day of change; it's the beginning of a process hopefully that will lead to change, to real change.
"The initial change has to come from the Cuban government."
Martinez, a Cuban native who immigrated to the U.S. at 15, said, "I have no hope that Raśl Castro -- who has been, frankly, the older brother's enforcer through most of the time they've been in power -- will be the kind of agent of change that Cuba needs today.
"What I think will happen is that we'll see, hopefully, in the future a new set of leaders that will come with new ideas."
Bush created his interagency commission in 2003 to "help hasten and ease Cuba's democratic transition," according to the group's Web site.
U.S. Secretary of State Condoleezza Rice is the commission's chair, and Commerce Secretary Carlos Gutierrez, a Cuban-American, is the co-chair.
"The work of the commission will ensure that the U.S. government is fully prepared, if asked, to assist a genuine Cuban transition government committed to democracy and which will lead to Cuba's reintegration into the inter-American system," a statement on the site says.
The commission report calls on the United States "to put in place preparations that will ensure that the U.S. will be in a position to provide technical assistance in the first two weeks after a determination that a Cuban transition is under way."
Such aid would include legal experts to help with elections. Training judges and police would be essential, too, according to the report.
The six months immediately after Castro's death or ouster would be key to determining U.S. success in the mission, the report says.
"This critical 180-day period could mean the difference between a successful transition period and the stumbles and missteps that have slowed other states in their transitions toward democracy," the report says.
It calls for an $80 million "democratic fund" for two years to strengthen civil society, boost opposition to Castro's regime and facilitate the free flow of information. It recommends at least $20 million a year for democracy programs "until the dictatorship ceases to exist."
The U.S. assistance is predicated on a request from a Cuban transitional government that is committed to a U.S. vision of Cuban democracy.
The report recommends offering a substantial aid package to the transitional government if it meets certain criteria under the 1996 Helms-Burton Act. Those criteria include freeing all political prisoners, legalizing all political activity, conducting democratic elections and establishing a free press. E-mail to a friend
CNN's Ed Payne contributed to this report.