Rio de Janeiro-based journalist Glenn Greenwald, a columnist for the Guardian newspaper who obtained secret files from NSA leaker Edward Snowden, told Globo on Sunday that a document dated June 2012 shows that Mexican President Enrique Pena Nieto's emails were being accessed.

That was a month before his election.

The NSA also intercepted some of Pena Nieto's voicemails. The communications included messages in which the future leader discussed the names of potential cabinet members.

A Mexican foreign ministry spokesman told he had seen the report but had no comment. A presidency spokesman could not immediately be reached for comment.

As for Brazil's Dilma Rousseff, the NSA said in the document that it was trying to better understand her methods of communication and interlocutors using a program to access all Internet content the president visited online.

Rousseff, who is due to make a state visit to Washington in October, held a working meeting to study the revelations in the Globo report, the channel said.

"If these facts prove to be true, it would be unacceptable and could be called an attack on our country's sovereignty," Justice Minister Jose Eduardo Cardozo said.

The NSA program allows agents to access the entire communications network of the president and her staff, including telephone, Internet and social network exchanges.

Cardozo met with US Vice President Joe Biden in Washington last week to discuss the matter.

The United States have rejected a Brazilian offer to negotiate a bilateral agreement on surveillance.

In July, Greenwald co-wrote articles in O Globo revealing that the US had a joint NSA-CIA base in Brazil to gather data on emails and calls flowing through the country.

Snowden, a former National Security Agency contractor, is now a fugitive in Russia under temporary asylum.

He is wanted by Washington on espionage charges linked to media disclosures about US surveillance programs.