Edward Snowden, the former contractor for the U.S. National Security Agency (NSA), has offered to help Brazil investigate the extent of NSA's spying against the South American country in an open letter published Tuesday.

Edward Snowden file photo

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.

But Snowden, who blew the whistle on the NSA's global surveillance scheme and is wanted in the U.S. for divulging secret documents, said, "Until a country grants permanent political asylum, the U.S. government will continue to interfere in my ability to speak."

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 U.S. 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 U.S. spying policy.

In conjunction with the publication of the letter, Snowden's supporters have organized an online drive to gather signatures to pressure Rousseff into giving him asylum.?