Brazil warned US Secretary of State John Kerry on Tuesday that failure to resolve the row over Washington's electronic spying could sow mistrust between the countries.
Foreign Minister Antonio Patriota told a press conference after talks with Kerry that revelations about the vast US global surveillance network posed a "new challenge in our bilateral relationship."
"If the implications of this challenge are not satisfactorily resolved, they ran the risk of casting a shadow of mistrust over our work," he added in Brasilia.
"Practices which harm the sovereignty and relations of trust between states and violate the individual freedoms which our countries so cherish must be stopped," Patriota said.
Kerry, who on his first trip to South America since he became US chief diplomat in February, said: "Brazil is owed answers with respect to those questions and they will get them."
"We will have this dialogue with the view to make it certain that your government is in complete understanding and complete agreement with what it is that we must to do provide security, not just for Americans, but for Brazilians and the people of the world," he added.
Washington has argued that it needs the vast surveillance program conducted by the National Security Agency to combat terrorism.
The US chief diplomat arrived here late Monday from Colombia where he also defended Washington's electronic espionage in the region, brought to light by fugitive intelligence leaker Edward Snowden.
Based on documents leaked by former NSA contractor Edward Snowden, the daily O Globo reported last month that Washington eavesdropped on Brazilians' telephone conversations and emails.
A US spy base in Brasilia, part of a worldwide network of 16 such stations operated by the NSA, also intercepted foreign satellite transmissions, it claimed.
The two foreign ministers also discussed Brazilian President Dilma Rousseff's scheduled state visit to the United States in October.
Last week, Patriota insisted that despite the furor over US snooping on phone calls and internet communications in Brazil, the trip was still on.
Kerry was scheduled to call on Rousseff later in the day.