The US ambassador to Brazil was summoned by authorities over new allegations that the US National Security Agency (NSA) spied on President Dilma Rousseff, an official said.

US Ambassador Thomas Shannon "was called to explain" the claims made by US journalist Glenn Greenwald, a Brazilian foreign ministry spokesman 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 on Monday.

Greenwald, a Guardian newspaper columnist who obtained secret files from NSA leaker Edward Snowden, has told Globo television that the agency snooped on the communications of Rousseff and Mexican President Enrique Pena Nieto.

Greenwald, who lives in Rio de Janeiro, told Globo's news programme "Fantastico'' that a document dated June 2012 shows that Mexican President Enrique Pena Nieto's emails were being read. The document is dated a month before Pena Nieto was elected.

If the facts of the report are confirmed, they would be considered very serious and would constitute a clear violation of Brazil's sovereignty.

Brazilian Justice Minister Eduardo Cardoza.

The document Greenwald based the report on includes communications from Pena Nieto indicating who he would like to name to some cabinet posts among other information. It's not clear if the spying continues.

In the case of Brazil's leader, the June 2012 document "doesn't include any of Dilma's specific intercepted messages, the way it does for Nieto,'' Greenwald told the Associated Press news agency in an email.

"But it is clear in several ways that her communications were intercepted, including the use of DNI Presenter, which is a programme used by NSA to open and read emails and online chats.''

Calls to Rousseff's office and a spokeswoman were not answered. Messages sent to a spokesman for Pena Pieto weren't immediately returned.

In July, Greenwald co-wrote articles in O Globo that said documents leaked by Snowden indicate Brazil was the largest target in Latin America for the NSA programme, which collected data on billions of emails and calls flowing through Brazil.

The Brazilian government denounced the NSA activities outlined in the earlier reports.

Greenwald began writing stories based on material leaked by Snowden in May, mostly for the Guardian newspaper in Britain.

Before news of the NSA programme broke, the White House announced that Rousseff would be honoured with a state dinner in October during a trip to the US, the only such full state dinner scheduled this year for a foreign leader. 

Source: Agencies