Maria das Gracas Silva Foster, CEO of Brazil's state-owned oil giant Petrobras, said Wednesday that alleged attempts by U.S. intelligence to spy on the country's strategic oil data have failed.

Addressing a senate hearing, Silva Foster said she was nevertheless concerned by revelations that the U.S. National Security Agency (NSA) intercepted digital communications between Brazilian President Dilma Rousseff and top aides, and those from the oil company.

So far, there are no signs that the NSA had managed to access or retrieve any information from the company, she said.

"To think we were breached, we have to have a record or evidence of that breach, and a sign that something was captured. For now, we don't have that," she told lawmakers.

"But that our name appears (in the spying revelations) generates concern about what would have been leaked," she added.

As Brazil prepares to hold an international bid on its deep- water oil reserves in October, officials are worried that information related to the bidding may have been compromised, but Silva Foster brushed away those concerns.

Data sent by Petrobras to the National Petroleum Agency (ANP), which is handling the bidding, was not violated because it is not relayed via the internet, she said.

"The data is sent to the ANP through a closed system within the (two) centers," she said, adding information is "sent physically, through CDs and DVDs, by hand."

Still, Petrobras plans to invest 21 billion reals (9.4 billion U.S. dollars) over a five-year period to boost data security, she said.

At a similar hearing on Tuesday, Magda Chambriard, director- general of the ANP, assured lawmakers there was no way to breach the agency's database.