NSA whistleblower Edward Snowden told the Brazilian government that he would be willing to help it investigate US eavesdropping activities in Brazil in exchange for political asylum.

In an open letter to the Brazilian people published by Folha de S. Paulo newspaper, Snowden - who is currently in hiding in Russia - offered support over NSA program's targeting of Brazil.

"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," said the letter, translated into Portuguese by the newspaper.

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

Reports that the US NSA spied on emails, phone calls, and text messages of President Dilma Rousseff provoked outrage in the South American nation.

The Brazilian government summoned US ambassador Thomas Shannon. Foreign minister Luiz Figueiredo expressed the "government's indignation" over "an inadmissible and unacceptable violation of Brazilian sovereignty".

Rousseff canceled a high-profile visit to Washington in October in retaliation.

Brazil is an important transit hub for trans-Atlantic fiber optic cables that are hacked.

The president encouraged the government to take several measures to counter-attack the NSA spying practice, including laying fiber optic lines directly to Europe and South American nations.

The letter was published after a US district judge ruled that the NSA's gathering of Americans' telephone records violates the US Constitution.