Cuban President Raúl Castro has reiterated his intention to step down two years from now, at age 86, and has said that in the future, all top Communist Party and other officials should be restricted to two five-year terms and should retire when they reach 70.

Calling for the party to inject more youth into its leadership, Castro pointed out that the experience of a number of countries has demonstrated that retaining aging leadership "is never positive."

"Never forget that during the final stage of the Soviet Union," a government he described as "esteemed and beloved," three first secretaries of the Communist Party Central Committee, all in their 70s, died within two years, he said.

Castro spoke Saturday at the opening session of the four-day Party Congress, an event held every five years to review national progress, plan for the future and recommit to "revolutionary" principals.

In his speech to the group Monday, Foreign Minister Bruno Rodriguez said President Obama's recent trip in Cuba was at least in part "a deep attack on our ideas, our history, our culture and our symbols." He accused Obama of trying to "dazzle" Cuba's growing private sector with outreach and promises that even the United States has said it hopes will lead to political change on the island.

Castro devoted much of his own two-hour address to the Cuban economy and its relationship to the political system. While he spoke less harshly than Rodriguez, Castro also referred to the United States as "the enemy." He said U.S. claims to have a multiparty political system were largely a sham, since both parties are based on money rather than popular will.

The United States "and their fellow travelers," Castro said, speak of a single-party system "as if it were a crime. They want to shape the world . . . adjust it to their own convenience."

Only U.S. methods have changed, not its goals, he said, and "we have to be more alert than ever."

As he has in the past, Castro said, "There have been concrete results in the dialogue and cooperation between the United States and Cuba" since the two governments announced in December 2014 that they would normalize relations.

"Nevertheless, the [U.S.] economic, commercial and financial blockade, in place for more than half a century, continues in effect, with unquestioned intimidating effects and extraterritorial scope, although we recognize that President Obama and senior administration officials oppose it and have repeatedly called on Congress to repeal it."

Obama's efforts to use his executive powers to bypass the embargo "are positive, but insufficient," Castro said, and the embargo "remains the principal obstacle" to Cuba's economic development. He also criticized the "illegal occupation" of Guantanamo Bay, and U.S. immigration policy that gives special preference to Cuban exiles in general and particular advantages to Cuban medical personnel willing to defect to the United States.

Since Castro took over from his brother Fidel in 2006, becoming president in 2008, he has significantly liberalized Cuba's state-owned economy. The percentage of Cuban workers employed by the state has shrunk from 81.2 percent in 2010 to 70.8 percent last year, he said, and more than half a million Cubans are now self-employed.

At the same time, he noted, the important tourism industry brought 3.5 million visitors to Cuba last year, the most ever. Since the last Party Congress in 2011, he said, Cuba has opened nearly 11,000 new hotel rooms and renovated an additional 7,000. More than 14,000 lodgings are now offered by private-sector homeowners who rent rooms and apartments to tourists.

Overall gross national product growth, Castro said, was 2.8 percent. But he criticized the party for failing to adequately plan for economic changes, including delays in eliminating Cuba's dual currency rates.

As part of the U.S. normalization, and Cuba's plans to grow the important tourism industry, U.S. airlines are expected to begin regular flights to Cuba later this year.

But plans to begin a ferry service between Miami and Havana have run into roadblocks. Carnival Corp. said Monday that Cuban law prevents Cuban-born passengers from using its services, effectively eliminating a major source of revenue for the route.

In a policy that stems from the early days of the Cuban revolution, when U.S.-sponsored exiles invaded the island, and the CIA tried to infiltrate Cuba to damage its economy and assassinate its leadership, Cuban-born people are only allowed to arrive there by air.

In a statement, Carnival said its 704-passenger Adonia luxury ship would begin sailing to Cuba every other week beginning May 1, the first time a cruise ship has sailed from the United States to Cuba in more than 50 years.

But "we want everyone to be able to go to Cuba with us," said Carnival chief executive Arnold Donald.

At least two Cuban Americans have filed lawsuits against the cruise line, charging illegal discrimination. In a speech in Miami last week, Secretary of State John F. Kerry said the Cuban government was "forcing its discrimination policy on us."

"If they want a full relationship and a normal relationship," he said, "they have to live by international law."