The letter, addressed to "the people of Brazil" and expected to be sent to authorities there, was published by Brazilian daily newspaper Folha de S. Paulo, which had previously run articles on the NSA's activities in Brazil, based on documents revealed by Snowden, Xinhua reported.
But Snowden, who blew the whistle on the NSA's global surveillance scheme and is wanted in the US for divulging secret documents, said, "Until a country grants permanent political asylum, the US government will continue to interfere in my ability to speak."
Snowden's political asylum expires in Aug next year
Snowden is currently living in Russia under temporary asylum, which is due to expire in August next year. He had previously requested asylum in Brazil, but that request remains unanswered.
The whistleblower said in the letter that several Brazilian senators have asked him for his help in investigating US spying on Brazil, but that Washington would continue to try to prevent him from speaking out until he is able to secure permanent asylum.
Soon after the spying revelations were first made, documents revealed Brazil was a main target of Washington's political and corporate spying, and Brazilian President Dilma Rousseff became one of the most outspoken critics of the US spying policy.
In conjunction with the publication of the letter, Snowden's supporters have organised an online drive to gather signatures to pressure Rousseff into giving him asylum.