In a letter, published on the website of the communist party newspaper Granma, Castro said that although he didn't "trust U.S. policies and have not exchanged a word with them, this does not mean however that I would oppose a peaceful solution to conflicts or threats of war."
Last month Cuba and the United States exchanged prisoners and agreed to discuss normalizing relations that had been trapped in over five decades of Cold War animosity.
While the U.S. economic trade embargo remains in place, President Barack Obama announced he would lift some sanctions and ease travel restrictions for U.S. citizens wishing to visit the island.
Despite the major shift in policy, for over a month Castro made no public comment on the deal, reigniting rumors that the 88-year-old former Cuban leader was in failing health.
Last week, Cuban and U.S. diplomats met to discuss re-establishing embassies in Washington and Havana.
While both sides hailed the first round of talks in Havana as productive, the negotiators sparred over issues like human rights and the discussions ended without signs of any breakthrough.
Talks are expected to resume in Washington, although no date has been set for the second round of discussions.
The U.S. broke relations with Cuba in 1961, amid fears that Castro and his bearded revolutionaries harbored communist sympathies.
Despite scores of CIA assassination attempts against him and a failed U.S.-backed invasion of Cuba, Castro stayed in power until 2006, when a still-undisclosed intestinal ailment forced him to step down.
Since then Castro rarely appears in public, instead opining on current events in occasional newspaper columns.
'Fighting until my last breath'
Raul Castro replaced him as president and demonstrated a less confrontational style of leadership than his older brother, who often led massive protests against U.S. policies.
The younger Castro has allowed more economic reforms to take place but resisted any challenges to the country's single-party communist style of government.
During the talks, Cuban officials said they hoped the United States and Cuba could learn to coexist despite the deep differences in the two countries political systems.
"We shall always defend the cooperation and friendship between all people, among them our political adversaries," Fidel Castro wrote in the message published Monday.
"With this spirit, I have fought," Castro concluded his letter, "and will continue fighting until my last breath."