The inter-agency effort, which also involves the Defense Department, recognises that the Cuba transition may not go peacefully and that the US may have to launch a nation-building exercise.

Caleb McCarry, the Cuba transition co-ordinator, is working on the project within the Office for Reconstruction and Stabilization, which was established by the Bush administration to prevent and prepare for post-conflict situations.

Every six months, the National Intelligence Council revises a secret watchlist of 25 countries in which instability could require US intervention. The reconstruction office, headed by Carlos Pascual - a Cuba-born former ambassador - was focused on Sudan, Haiti, Congo and Nepal. In a controversial move, Cuba was added to the list.

The US Institute of Peace, funded by Congress to work on conflict management, declined to lend its expertise to the Cuba project. "This was an exercise in destabilisation, not stabilisation," said one person involved.

Mr McCarry acknowledges wearing two hats: to help a post-Castro Cuba establish a democratic government and market economy, and to hasten that transition.

Condoleezza Rice, secretary of state, appointed Mr McCarry in July. His post was recommended by the Commission for Assistance to a Free Cuba, which she noted was created by President George W. Bush "to accelerate the demise of Castro's tyranny".

The commission declared in its May 2004 report that it "sought a more proactive, integrated and disciplined approach to undermine the survival strategies of the Castro regime and contribute to conditions that will help the Cuban people hasten the dictatorship's end".

Wholesale engagement is envisaged post-Castro, including immediate assistance so that "schools are kept open and provided with new instructional material and staff", food and medical aid is distributed, and pensions are paid.

Mr McCarry told the FT that last year's tightening of the US economic embargo - such as restrictions on visits to Cuba by Cuban-Americans, and a curbing of remittances - had cost the Castro regime an estimated $500m (€417m, £283m) in lost income.

Human Rights Watch last month condemned the travel restrictions imposed by both Cuba and the US, saying: "Both countries are sacrificing people's freedom of movement to promote dead-end policies." Mr McCarry declined to comment on his work in the Office for Reconstruction and Stabilization, except to say that it would be "thoughtful and respectful of the Cuban people and their wish to be free".

"The transition genie is out of the bottle," he said, referring to opposition activities inside Cuba, and a "broad consensus" reached with the exiled community. "They are the ones to define a democratic future for Cuba."

Officials say the US would not "accept" a handover of power from Mr Castro, who is 79, to his brother Raul, aged 74. While it is not clear what the US position means, Mr McCarry stressed the US would not "impose" its help.

Addressing the Association of the US Army last month, Mr Pascual indicated his co-operation with the military was at an early stage. He said his strategic planning was aimed at understanding "how we would manage that transition process between Fidel's death and a democratic Cuba, because we know that at some point, that is going to happen".

Analysts said the military, worried about a mass exodus of Cuban refugees, was keen to understand the administration's plans for what is called "the day after".

But they also question whether the White House is really committed to the task, noting the limited budgets of both Mr Pascual and Mr McCarry.

Some suspect Mr Bush drew attention to the issue in 2004 with an eye on securing votes in Florida from Cuban exiles. "The US has a history of not being very successful in achieving desired outcomes in Cuba," cautioned Daniel Erikson, analyst with the Inter-American Dialogue think-tank.

A US military officer said: "The truth is that nobody, including anyone on the island, knows what will happen during a transition. It's a little like trying to figure out how many angels can dance on the head of a pin."