Brazil-based American reporter Glenn Greenwald said Tuesday that he had received more than 15,000 secret US government documents from intelligence leaker Edward Snowden.

Greenwald, who revealed details of Washington's vast electronic surveillance program in Britain's Guardian newspaper, gave no details of the content of the files as he testified before the Brazilian Senate´s foreign relations committee.

"I did not do an exact count, but he gave me 15,000, 20,000 documents. Very, very complete and very long," Greenwald said, responding to questions from lawmakers.

"The stories we have published are a small portion. There will certainly be more revelations on the espionage activities of the US government and allied governments (...) on how they have penetrated the communications systems of Brazil and Latin America," he said.

He added that he could not give more details on the documents because an investigation of Snowden is under way in the United States.

"The pretext (given by Washington) for the spying is only one thing: terrorism and the need to protect the (American) people. But the reality is that there are many documents which have nothing to do with terrorism or national security, but have to do with competition with other countries, in the business, industrial and economic fields," Greenwald said.

Referring to the XKeyscore secret surveillance system allegedly used by NSA to monitor Internet traffic, he said Washington was collecting not just metadata, "but also the content of emails and what is being discussed in telephone conversations. It is a powerful program which frightens."

In an interview with AFP last month, Greenwald explained that Snowden personally handed him the documents in Hong Kong, where the former NSA contractor temporarily took refuge, after fleeing the United States.

The Brazilian O Globo magazine carried stories alleging that Washington maintained a spy center in Brasilia which was part of a network of 16 such facilities around the world to intercept foreign satellite transmissions.

Last month, US Vice President Joe Biden called Brazilian President Dilma Rousseff to provide a explanation following press reports of US electronic surveillance in Brazil based on leaks from Snowden, a former US National Security Agency contractor.

Snowden is now a fugitive from US justice and is currently living at an unknown location in Russia after Moscow granted him temporary asylum for a year on August 1.