"We take his reference of 'international norms and principles' as a positive sign," State Department spokeswoman Jen Psaki told reporters, after Castro's letter was released.
Psaki said Washington now looked forward "to the Cuban government implementing those international norms and principles for a democratic, prosperous and stable Cuba."
The 88-year-old and still influential former leader released his letter to state media late yesterday, after last week's historic first round of talks on restoring diplomatic ties between America and Cuba.
And his language contained a nod to a normalization in ties.
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"Any peaceful and negotiated solution to problems between the United States and the peoples or any people of Latin America, which does not imply force or the use of force, should be treated according to international norms and principles," Castro said.
In December, US President Barack Obama and Raul Castro, who succeeded an Fidel as Cuba's president in 2006, agreed to begin normalizing ties more than 50 years after they were broken