Brazil and Mexico have both demanded an explanation from the US over claims that the National Security Agency (NSA) spied on their presidents.
Internet data from Brazilian leader Dilma Rousseff and Mexico's Enrique Pena Nieto was intercepted, journalist Glenn Greenwald told Brazil's TV Globo.
Mr Greenwald obtained secret files from US whistle-blower Edward Snowden.
Brazil said data interception would represent an unacceptable violation of sovereignty. Mexico called for a probe.
"You cannot allow... a US agency, or someone that has been hired by the US government, to follow what any Brazilian citizen is doing," Brazilian Senator Eduardo Suplicy told the BBC's Newshour programme.
Both the Brazilian and Mexican governments summoned their respective US ambassadors. Mexico requested an "an exhaustive investigation" to determine who may be responsible for the alleged spying on Mr Pena Nieto's emails before his election last year, the AFP news agency reported.
In July, Brazilian newspaper O Globo reported that the US had seized web traffic and phone calls across the region.
'Attack on sovereignty'
Mr Greenwald, a columnist for the British Guardian newspaper, told TV Globo's news programme on Sunday - "Fantastico" - that secret documents leaked by Edward Snowden showed how US agents had spied on communications between aides of Brazil's President Dilma Rousseff.
Brazil's Justice Minister Jose Eduardo Cardozo said that "if these facts prove to be true, it would be unacceptable and could be called an attack on our country's sovereignty".
According to the report, the NSA also used a program to access all internet content that Ms Rousseff visited online.
Her office said the president was meeting top ministers to discuss the case.
The BBC's Julia Carneiro in Sao Paulo says there is a suspicion in Brazil that the US is spying on its government's communications because of commercial interests.
The revelations come at sensitive moment for Ms Rousseff, who due to make her first state visit to the United States in October, our correspondent says.
It is the only state visit offered by President Barack Obama to a foreign leader this year.
Brazilian officials have not said whether the allegations could lead Ms Rousseff to call off her visit to Washington.
The report also alleges that the NSA monitored the communications of Mexican President Enrique Pena Nieto, even before he was elected in July last year.
Mr Greenwald said that a document dating from June 2012 showed that Mr Pena Nieto's emails were being read.
The documents were provided to Mr Greenwald by ex-US intelligence contractor Edward Snowden, who was granted temporary asylum in Russia after leaking secret information to media in the US and Britain.
Mr Greenwald was the first journalist to reveal the secret documents leaked by Mr Snowden on 6 June. Since then, he has written a series of stories about surveillance by US and UK authorities.
The detention last month for nine hours at London's Heathrow airport of Mr Greenwald's partner, David Miranda, caused widespread controversy in the UK and abroad.
Mr Greenwald said the detention of his partner amounted to "bullying" and was "clearly intended to send a message of intimidation" to those working on the NSA revelations.
The British government said that it was right for the police to act if they believed that someone had "highly sensitive stolen information".