During his breakthrough visit to Cuba in December 2014, Obama managed to cause resentment inside the Cuban government by a speech broadcast directly into Cuban homes calling for more political freedom and democracy in the one-party state.
Castro said that one-party Communism is essential to defend the political system of Cuba.
"If one day they manage to fragment us, that would be the beginning of the end of the revolution, of socialism and independence in our homeland," Castro told the delegates gathered for the congress.
"We are not naive, and we are aware of powerful external forces that aspire to, as they say, 'empower' non-state actors to generate agents of change and finish off the revolution by other means," said Castro, implying the US is trying to turn Cuba's growing number of self-employed people into an opposition.
As yet another example of "new methods", Castro mentioned US migration policies that encourage Cubans to defect were "a weapon against the revolution." Migration has surged since the 2014 detente as Cubans take advantage of a US policy that grants them citizenship as soon as they arrive.
Castro reiterated the party's commitment to the reforms which he said should be implemented faster. But he said Cuba was not moving towards capitalism, citing China and Vietnam as models, while emphasizing that social ownership and cooperatives were mostly preferable to private property.