Both sides are resorting to much stronger and less diplomatic language
Cuba's leaders do not want to normalise ties with the US because then they would lose their excuse for the state of the country, says Hillary Clinton.
Cuba's response to recent US efforts to improve relations had revealed "an intransigent, entrenched regime" in Havana, said the US secretary of state.
The Cuban authorities have long blamed a 48-year US trade embargo for holding back the country's development.
The US says the embargo will remain until Cuba improves human rights.
Relations between Washington and the communist government in Havana have soured in recent months after early expectations of an improvement under the Obama administration.
The BBC's Michael Voss in Havana says initial hopes of improved relations are receding with both sides resorting to much stronger and less diplomatic language.
Mrs Clinton said the response of Cuban President Raul Castro and his brother, ex-leader Fidel Castro, to US efforts to improve ties proved they had no interest in political reform or ending the sanctions.
There should be an opportunity for a transition to a full democracy in Cuba... but it may not happen any time soon
Hillary Clinton US Secretary of State
"It is my personal belief that the Castros do not want to see an end to the embargo and do not want to see normalisation with the United States, because they would lose all of their excuses for what hasn't happened in Cuba in the last 50 years," she said in a speech at Kentucky's University of Louisville.
"I find that very sad, because there should be an opportunity for a transition to a full democracy in Cuba and it's going to happen at some point, but it may not happen any time soon."
Earlier this month, Cuban Foreign Minister Bruno Rodriguez held a meeting with Cheryl Mills, Mrs Clinton's chief-of-staff, in one of the highest level contacts between the two countries for years.
US officials said the two "did not agree on very much" at the talks, which were held in New York on the sidelines of a UN forum on aid for quake-hit Haiti.
The recent death of the jailed dissident hunger striker, Orlando Zapata, brought widespread international condemnation and has focused attention on Cuba's human rights record.
The authorities here have responded by going on the offensive.
In a televised speech last weekend, President Raul Castro accused the US, Europe and the Western media of waging an unprecedented publicity war against the island.
"The empire and its allies have launched a new crusade to try to demonise Cuba and to destabilise the country," a front page editorial in the communist party newspaper Granma added.
The authorities have now called for a massive May Day mobilisation to denounce the West and in support of the revolution.
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