By JOSE DE CORDOBA

Retired dictator Fidel Castro slapped down his younger brother, Cuba's President Raúl Castro, six days after he suggested Cuba was ready to negotiate fundamental differences with the U.S.

The brusque rebuke sent a clear, two-part message to the U.S.: Despite Raúl Castro having assumed Cuba's presidency last year, it's the elder Castro who continues to be in charge, especially on relations with the U.S. And the U.S. shouldn't expect Cuba to reciprocate any conciliatory actions taken by Washington.

"Fidel is calling the shots, at least on relations with the U.S.," said Brian Latell, a former Central Intelligence Agency Cuba analyst who has written a biography of Raúl Castro. "And he has rejected in intransigent language President Obama's overture that Cuba signal a willingness to move forward by making a concession."

Secretary of State Hillary Clinton, testifying before the House Foreign Affairs Committee, addressed Fidel Castro's response saying, "I think that the president's actions did draw a response from Raúl Castro -- which was then contradicted today by Fidel Castro," signaling that a debate has started within Cuba. "I mean, this is a regime that is ending." She said the U.S. has "responded to Raúl Castro's comments by saying that we would consider a discussion that would include human rights and political prisoners."

The elder Castro's rebuke to President Barack Obama and to his younger brother came in one of his periodic trademarked commentaries published in Cuba's official newspaper, Granma.

"Without a doubt, the President wrongly interpreted Raúl's statement," wrote the elder Castro, 82, referring to Mr. Obama's positive reaction to a speech given by younger brother Raúl, 77, in Venezuela before the Americas summit of hemispheric nations.

The speech -- in which Raúl Castro said Cuba was ready to discuss "human rights, freedom of expression, political prisoners: everything, everything, everything" -- set off speculation the two countries could quickly settle their differences. Since 1962, the U.S. has maintained a trade embargo. This month, Mr. Obama lifted restrictions on travel and remittances sent by Cuban-Americans to relatives on the island.

Write to José de Córdoba at jose.decordoba@wsj.com