The US government spied on Brazilian state-run oil giant Petrobras, according to intelligence documents released by Globo television.

Globo said it obtained the information from Glenn Greenwald, a columnist for the Guardian newspaper who obtained secret files from US National Security Agency leaker Edward Snowden.

A week ago, the network reported that the NSA had intercepted communication from the presidents of both Brazil and Mexico, a report also based on information from Greenwald.

No details were immediately available about the nature of the spying, other than that the information was based on documents dated June 2012.

Globo said it was unaware of the scope or objectives of the spying on Petrobras, the world's leader in deep-water oil exploration.

Petrobras has an annual turnover of 200 billion reais (about $90 billion).

Obama has vowed to ease tensions with Brazilian President Dilma Rousseff and Mexican President Enrique Pena Nieto, telling them in separate meetings at last week's G20 summit that he understood their reaction.

The US president has vowed to provide answers by Wednesday.

The Brazilian leader, who is scheduled to visit Washington on October 23, has warned that she will cancel the trip if she does not receive convincing explanations from the Obama administration.

Greenwald said the NSA was using a program to access all Internet content Rousseff visited online in order to better understand her methods of communication and interlocutors.

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

Some of Pena Nieto's email, phone calls and text messages were intercepted, including communications in which he discussed potential cabinet members before he was elected in July 2012, according to Greenwald.

Brazilian Foreign Minister Luiz Alberto Figueiredo said that, if proven, the report that Rousseff was spied on "represents an unacceptable and impermissible violation of Brazilian sovereignty".

Communications Minister Paulo Bernardo said the scope of the espionage was broader and more serious than had initially been thought and that US explanations had so far been "false."