Two American journalists known for their investigations of the US government have teamed up to report on the National Security Agency's role in what one called an "assassination programme".
The journalists provided no evidence of the purported programme at the news conference, nor details of who it targeted.
Jeremy Scahill, a contributor to The Nation magazine and best-selling author of Dirty Wars, will be working with Glenn Greenwald, the Rio-based journalist who has written stories about US surveillance programmes based on documents leaked by former NSA contractor Edward Snowden.
Scahill spoke to moviegoers in Rio de Janeiro, where the documentary based on his book was given its Latin American premiere at the Rio Film Festival.
He said: "The connections between war and surveillance are clear. I don't want to give too much away.
"But Glenn and I are working on a project right now that has at its centre how the National Security Agency plays a significant, central role in the US assassination programme.
"There are so many stories that are yet to be published that we hope will produce 'actionable intelligence', or information that ordinary citizens across the world can use to try to fight for change, to try to confront those in power."
The Dirty Wars documentary, directed by Richard Rowley, traces Scahill's investigations into the Joint Special Operations Command.
Video: See the trailer for Jeremy Scahill's film, 'Dirty Wars'
The movie, which won a prize for cinematography at the Sundance Film Festival in Utah, US, follows Scahill as he travels around the globe, from Afghanistan to Yemen to Somalia, talking to the families of people killed in US strikes.
Neither Scahill nor Greenwald, who also appeared at the film festival's question and answer panel, provided many details about their joint project.
Greenwald has been making waves with a series of stories on the NSA spying programme that appeared in British newspaper The Guardian.
Brazilian President Dilma Rousseff postponed a scheduled state dinner with Barack Obama after television reports to which Greenwald had contributed revealed that US spy programmes had aggressively targeted the Brazilian government and private citizens.
Rousseff railed against the US surveillance during her address to the United Nations General Assembly last week.
Both Scahill and Greenwald applauded Rousseff's reaction to the revelations.
But Greenwald warned: "The really important thing to realise is the desire for surveillance is not a uniquely American attribute.
"America has just devoted way more money and way more resources than anyone else to spying on the world."