HAVANA (AP) ― President Raul Castro delivered a full-throated defense of Cuba’s one-party political system on Sunday, and a sharp warning to Communist Party delegates to fight corruption he said was a greater threat to the revolution than anything the United States could dream up.

In a stern closing speech to the party’s national conference, Castro reiterated a pledge to institute term-limits for Cuban officials, saying a constitutional amendment would be required but that leaders should begin to adopt the practice even before it is formalized.

Castro has spoken previously about limiting high-ranking officials including himself to two, 5-year terms.

The U.S. threat to Cuba and the limits it placed on reform was a continuing theme of the speech. Cuba’s president upbraided those who were hoping to see more fundamental changes come out of the two-day meetings ― or any new faces amid the aged upper ranks of the party and government hierarchy.

“There has been no shortage of criticism and exhortations by those who have confused their intimate desires with reality, deluding themselves that this conference would consecrate the beginning of the dismantling of the political and social system the revolution has fought for more than half a century,” he said.

The Cuban leader said those who want to see Cuba restore a multiparty system are forgetting that it is under siege from a Goliath to the north that would stop at nothing to destroy it.

“To renounce the principle of a one-party system would be the equivalent of legalizing a party, or parties, of imperialism on our soil,” he said.

Castro was sharply critical of the United States’ democratic system, which he said only concentrated power in the hands of the wealthy. He said that while Cuba had only one party, it sought the participation of all citizens through party and workplace meetings.

“We must promote democracy in our society, starting with the party,” he said, urging rank-and-file members to speak up when they disagree with something.

The speech included denunciations of Washington’s 50-year trade embargo, its support for dissidents and its imprisonment of Cuban agents who had infiltrated anti-Castro groups in Miami.

Castro also poured water on hopes that a new generation of Cuban politicians were any closer to the brass ring of power, saying the island remained without a backbench of young leaders.

The conference was presided over by the 80-year-old Castro and his 81-year-old chief deputy, Jose Ramon Machado Ventura. The island’s third ranking leader, Ramiro Valdes, is 79.

Castro and his brother Fidel, now retired, have ruled Cuba since their 1959 revolution. There was no sign of the elder Castro at the confab, which was closed to foreign journalists.

Raul Castro has pushed a series of dramatic economic reforms since taking power in 2008, legalizing the sale of private homes and used cars, allowing hundreds of thousands to go into business for themselves, turning fallow government land over to small-time farms, and extending bank loans to entrepreneurs and others.

But many social and political reforms have not materialized. After promising in July to study changes to immigration laws that keep most Cubans from ever leaving the country, Castro told the nation in December that the time was not yet right, citing the continued threat from Washington.

At a Communist Party Congress in April, Castro and brother Fidel raised hopes that a new generation of leaders would soon appear on the horizon. Nine months later, there have been few visible changes.

A Cuban official told the Associated Press recently that despite the lack of movement among cabinet ministers and other senior leaders, many midlevel government posts have quietly changed hands, with younger officials moving up. The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly, and his assertions could not be independently confirmed.

Castro spent a large part of his 40-minute speech warning delegates about the evils of corruption, saying graft was “the principal enemy of the revolution, much more damaging than the multimillion-dollar subversive and interfering programs of the U.S. government and its allies.”

He said the Interior Ministry was in the midst of several high-profile investigations of graft and other violations, which would become known at the appropriate time.

“To win the battle against corruption we must first stop it and then liquidate it,” Castro said. “We have warned that within the law, we will be implacable.”