NSA whistleblower Edward Snowden has offered to help the Brazilian government in its investigations around US spying in exchange for permanent asylum in the country.

In an open letter published by Brazilian newspaper Folha de São Paulo today (17), the former NSA contractor suggests - without addressing president Dilma Rousseff directly - that his ability to speak is limited in Russia, where is temporarily based.

Snowden also says that this situation can only change and he will only be able to assist governments that have been spied on such as Brazil until a country grants him permanent asylum. Back in July, Snowden sent asylum requests to 21 countries including Brazil and other Latin American nations - at the time, the Brazilian government said it would not respond to the request.

Snowden seems confident that the events that have taken place since his initial request was denied mean his assistance is more valuable to Brazil than ever before.

"These [NSA] programs were never about terrorism: they're about economic spying, social control, and diplomatic manipulation. They're about power," Snowden says.

"Many Brazilian senators agree, and have asked for my assistance with their investigations of suspected crimes against Brazilian citizens. I have expressed my willingness to assist wherever appropriate and lawful, but unfortunately the United States government has worked very hard to limit my ability to do so," he adds.

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

In his open letter, Snowden says that the NSA spying activities "threatens to become the greatest human rights challenge of our time" and says that the episodes involving the monitoring of president Rousseff's communications as well as oil company Petrobras, were a breach of privacy despite the US attempts to describe it as something that was needed to ensure safety.

"They [the NSA] did it without asking the public in any country, even their own," Snowden adds.

Snowden's original asylum request to Dilma Rousseff's government was declined only a few days before it emerged that Brazil was also on the list of countries being spied. President Rousseff only found out that her own communications were being monitored in September.

In his open letter to Brazil, Snowden says that Brazil's reaction to these revelations - which included a whole host of measures including the creation of a national email system , controversial demands that data is hosted locally and laying fibre-optic lines directly to Europe and South American nations to avoid use of the US-centric internet backbone which is said to have facilitated NSA spying - has been "inspiring" and that has influenced his decision to ask for asylum again.

"If Brazil hears only one thing from me, let it be this: when all of us band together against injustices and in defense of privacy and basic human rights, we can defend ourselves from even the most powerful systems," the whistleblower says.

Read Edward Snowden's full open letter to the people of Brazil here.