SAO PAULO (Reuters) - The U.S. government allegedly spied on Brazilian state-run oil company Petroleo Brasileiro SA, known as Petrobras, according to the web site of Globo, Brazil’s biggest television network.

The network, which a week ago aired a report alleging that the U.S. National Security Agency intercepted communications by the presidents of Brazil and Mexico, said its information again came from Glenn Greenwald, an American activist who has worked with fugitive former NSA analyst Edward Snowden to expose the extent of U.S. spying at home and abroad.

Promotional teasers from the network said it would give details of the spying on Sunday night, again on its “Fantastico” program.

Greenwald, a blogger and civil liberties activist who lives in Rio de Janeiro, declined to discuss the Petrobras allegations until after the program airs. Petrobras officials could not be reached for comment.

New revelations of U.S. spying could complicate a diplomatic row between the United States and Brazil sparked by the allegations of NSA spying on the private phone calls and emails of Brazilian President Dilma Rousseff.

Brazil has demanded a formal apology and Rousseff aides have said the issue could derail a state visit she is due to make to the United States in October.

The tensions last week led to an impromptu meeting between Rousseff and U.S. President Barack Obama on the margins of the G20 meeting in Russia. Obama said he would investigate the allegations.

Any spying on Petrobras, which has discovered some of the world’s biggest offshore oil reserves in recent years, is sure to raise hackles in Brazil, a country that has long been suspicious of foreign designs on its abundant natural resources.

Brazil’s so-called sub-salt polygon, where many of the new finds have been discovered, may contain as much as 100 billion barrels of oil, according to Rio de Janeiro State University. One find alone, the giant Libra field, has estimated reserves of up to 12 billion barrels of oil, or enough to supply all U.S. oil needs for nearly two years.