Edward Snowden has offered to help Brazil investigate US spying on its soil in exchange for political asylum, in an open letter from the NSA whistleblower to the Brazilian people published by the Folha de S Paulo newspaper.

"I've expressed my willingness to assist where it's appropriate and legal, but, unfortunately, the US government has been working hard to limit my ability to do so," Snowden said in the letterwrote.

"Until a country grants me permanent political asylum, the US government will continue to interfere with my ability to speak out."