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 two countries, AFP reports.
Brazil was outraged by media reports of widespread US phone and Internet eavesdropping based on information leaked by fugitive intelligence contractor Edward Snowden.
Foreign Minister Antonio Patriota told reporters 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 run the risk of casting a shadow of mistrust over our work," he added.
"Practices which harm the sovereignty and relations of trust between states and violate the individual freedoms which our countries so cherish must be stopped."
Kerry, who is on his first trip to South America since he assumed his post 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," Kerry added.
US officials have defended the surveillance programs as entirely lawful measures that have helped foil dozens of terrorist attacks around the world.
Brazil, Latin America's economic powerhouse, has meanwhile sought to assert regional independence from Washington.
Kerry arrived here late Monday from Colombia, where he also defended Washington's electronic espionage.
"I think it's very obvious to everybody that this is a dangerous world we're living in," Kerry told reporters in Bogota on Monday.
"We are necessarily engaged in a very complex effort to prevent terrorists from taking innocent lives in many different places."
Based on documents leaked by Snowden, the daily O Globo reported last month that Washington eavesdropped on Brazilians' phone calls and Internet communications.
A spy base in Brasilia, part of a worldwide network of 16 such stations operated by the US National Security Agency, also intercepted foreign satellite transmissions, it claimed.
O Globo also published an NSA document which seemed to indicate that the Brazilian embassy in Washington and the Brazilian mission to the United Nations in New York were targeted by the US spy agency.
Snowden, who was granted asylum in Russia on August 1 after spending over five weeks in a Moscow airport transit zone, is said to now be at an undisclosed location in the country.
The United States wants to put him on trial for leaking sensitive US secrets but Moscow has steadfastly refused to hand him over.
In fence-mending remarks, Kerry took pains to acknowledge emerging Brazil's growing international profile.
"The United States recognizes and welcomes and greatly appreciates the vital leadership role, the increasing leadership role, that Brazil plays on the international stage," he said.
He cited Brasilia's participation in global peace initiatives, its promotion of human rights and its efforts to help maintain peace in some parts of the world, notably Haiti.
"We're also exploring opportunities for closer collaboration on peacekeeping in Africa," Kerry said.
Kerry and Patriota also discussed Brazilian President Dilma Rousseff's scheduled state visit to the United States in October. Kerry met with Rousseff later in the day.