The US National Security Agency's (NSA) spy programme targeted the communications of the Brazilian and Mexican presidents, according to documents leaked by Edward Snowden.
Journalist Glenn Greenwald, to whom Snowden passed the documents, said a paper dated June last year shows that Mexican President Enrique Pena Nieto's e-mails were being read.
The document's date is a month before Pena Nieto was elected and includes communications from Pena Nieto indicating who he would like to appoint to government posts.
Greenwald also said communications from Brazil's leader Dilma Rousseff were intercepted, "including the use of DNI Presenter, which is a programme used by the NSA to open and read emails and online chats".
The US targeting mapped out the aides with whom Rousseff communicated and went a level further by tracking patterns of how those aides communicated with one another and also third parties, Greenwald said.
Brazilian Justice Minister Eduardo Cardozo said that "if the facts are confirmed, they would be considered very serious and would constitute a clear violation of Brazil's sovereignty".
Cardozo added: "This is completely outside the standard of confidence expected of a strategic partnership, as the US and Brazil have."
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.
The Brazilian government denounced the NSA activities outlined in the earlier reports.
Greenwald, who lives in Rio de Janeiro, began writing stories based on material leaked by Snowden in May, mostly for The Guardian newspaper in Britain.