Brazil has said it is not considering granting asylum to Edward Snowden even after the former US National Security Agency contractor offered to help investigate revelations that the NSA has spied on Brazilians and their president.

In a letter published in a leading Brazilian newspaper on Tuesday, Snowden said he will work with Brazilian authorities to investigate the case, in exchange for protection from possible extradition to the US.

The Brazilian government should grant him asylum and the US government must understand that the NSA violated rights protected in Brazil's Constitution. Brazilian Senator Eduardo Suplicy

"I have expressed my willingness to assist wherever appropriate and lawful, but unfortunately the US government has worked very hard to limit my ability to do so," the letter published in the Folha de S. Paulo newspaper said.

But the Brazilian government said it has not received an official asylum request from Snowden since his arrival in Moscow in June, a foreign ministry spokesman said, adding that without an official request, asylum could not be considered.

Avaaz, a website for public petitions, also launched a signature campaign to press Brazil's President Dilma Rousseff to grant Snowden asylum. The only person who can grant Snowden's request is Rousseff.

The revelation that the NSA was spying on Brazil damaged relations between the US and Latin America's largest country.

Rousseff has since become an advocate for curbing Internet surveillance.

Support for Snowden

Evidence that the NSA monitored Rousseff's email and cell phone, and hacked into the computing network of state-run oil company Petrobras, angered Brazilians and led to a probe into US spying.

Some members of Brazil's Congress have expressed a desire to interview Snowden.

In a Twitter message, Brazilian Senator Ricardo Ferraco, chairman of the Senate foreign relations committee, said "Brazil should not miss the opportunity to grant asylum to Edward Snowden, who was key to unravelling the US espionage system."

"The Brazilian government should grant him asylum and the US government must understand that the NSA violated rights protected in Brazil's Constitution," fellow committee member Senator Eduardo Suplicy said.

Snowden is living in Russia under temporary asylum that will expire in August.

He had previously asked Brazil to grant him asylum, but the country did not reply to his request.

Al Jazeera's Rachel Levin, reporting from Natal, Brazil, said opinion on Snowden is divided.

On Monday, White House spokesman Jay Carney dismissed the suggestion that the US could grant amnesty to Snowden if he were to turn over the documents in his possession.