U.S. National Security Agency leaker Edward Snowden has said he would be willing to help Brazil's government investigate American spying on its soil - but only if he is granted political asylum, the Associated Press said.

Snowden said in a letter to a newspaper that he has been impressed by the Brazilian government's strong criticism of the massive National Security Agency (NSA) spy programme that targets internet and telecommunications around the globe.

The programme included monitoring the mobile phone of Brazilian president Dilma Rousseff. Brazilian senators have asked for Snowden's help during hearings about the NSA programme's aggressive targeting of Brazil, an important transit hub for transatlantic fibre optic cables that are hacked.

He said in the letter: "I've expressed my willingness to assist where it's appropriate and legal, but, unfortunately, the U.S. government has been working hard to limit my ability to do so. "Until a country grants me permanent political asylum, the U.S. government will continue to interfere with my ability to speak out."

The Guardian newspaper first published accounts of the NSA's spy programmes in June, based on some of the thousands of documents Snowden handed over to Brazil-based American journalist Glenn Greenwald and his reporting partner Laura Poitras, an American film-maker.

Ms Rousseff cancelled an October visit to Washington that was to include a state dinner. She has joined Germany in pushing for the United Nations to adopt a symbolic resolution which seeks to extend personal privacy rights to all people.

She has also ordered her government to take several measures, including laying fibre optic lines directly to Europe and South American nations, in an effort to "divorce" Brazil from the U.S.-centric backbone of the internet that experts say has facilitated NSA spying.

The Snowden letter was published one day after a U.S. district judge ruled that the National Security Agency's bulk collection of millions of Americans' telephone records probably violates the U.S. constitution's ban on unreasonable search. The case is likely to go all he way the U.S. Supreme Court for a final decision.