In a letter, he wrote: “I don’t trust the policy of the US, nor have I exchanged a word with them, but this does not mean I reject a peaceful solution to conflicts.”
These were his first comments since December’s historic move by Cuba and the US after decades of hostilities.
High-level talks were held last week.
In the letter published by the state-run newspaper Granma, Mr Castro wrote: “We will always defend cooperation and friendship with all the people of the world, including with our political adversaries.”
The 88-year-old appeared to be backing decisions taking by his younger brother Raul – the current Cuban president who succeeded him in 2008.
The Cuban president, the letter stated, “has taken the pertinent steps in accordance with his prerogatives and the powers given to him by the National Assembly the Communist Party of Cuba”.
It was hardly a ringing endorsement of US-Cuban rapprochement but nor was it a rejection of the decision to re-establish political ties with the great enemy to the north, the BBC’s Will Grant in Havana reports.
Fidel Castro has spent his whole adult life in conflict with the US, and it should come as little surprise that he has not given a glowing reception now those hostilities are apparently at an end, our correspondent says.
That said, he adds, many in Cuba believe that the current efforts to warm the long-frozen relations would not have started in the first place unless Fidel had given them his benign approval.
His silence on the issue had led to growing speculation over his health.
Earlier this month, Fidel Castro sent a letter to former footballer Diego Maradona to quash rumours that he had died.