The US journalist who published documents leaked by fugitive former US intelligence contractor Edward Snowden plans to make new revelations shortly.

Glenn Greenwald has said the revelations on secret US surveillance of the internet would be made "within the next ten days or so".

He made the comments at a Brazilian congressional hearing that is investigating US internet surveillance in Brazil.

"The articles we have published so far are a very small part of the revelations that ought to be published," he said in Portuguese.

"There will certainly be many more revelations on spying by the US government and how they are invading the communications of Brasil and Latin America," he added.

Mr Snowden is said to have passed up to 20,000 classified documents from the National Security Agency to the journalist.

The Rio de Janeiro-based columnist for Britain's Guardian newspaper said he has recruited the help of experts to understand the documents.

He said some of the documents were "very long and complex and take time to read".

Mr Greenwald told Reuters he does not believe the pro-transparency website WikiLeaks had obtained a package of documents from Mr Snowden, and that only he and filmmaker Laura Poitras have complete archives of the leaked material.

After a meeting in June with Mr Snowden in Hong Kong, Mr Greenwald published the first of many reports that rattled the US intelligence community by disclosing the breadth and depth of alleged NSA surveillance of telephone and internet usage.

Mr Greenwald said Mr Snowden was pleased that he had stirred up a worldwide debate on internet privacy and secret US surveillance programmes used to monitor emails.

He also said Mr Snowden, who is wanted by US authorities, was happy to leave a Moscow airport after being granted temporary asylum.

"I speak with him a lot since he left the airport, almost every day. We use very strong encryption to communicate," Mr Greenwald told the Brazilian legislators. "He is very well."

Mr Snowden was in hiding in Hong Kong before flying to Russia in late June.