Two American journalists known for their investigations of the United States' government said Saturday they've teamed up to report on the National Security Agency's role in what one called a "U.S. assassination program."

The journalists provided no evidence of the purported U.S. program at the news conference, nor details of who it targeted.

Jeremy Scahill, a contributor to The Nation magazine and the New York Times best-selling author of "Dirty Wars," said he will be working with Glenn Greenwald, the Rio-based journalist who has written stories about U.S. surveillance programs based on documents leaked by former NSA contractor Edward Snowden.

"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 center how the National Security Agency plays a significant, central role in the U.S. assassination program," said Scahill, speaking to moviegoers in Rio de Janeiro, where the documentary based on his book made its Latin American debut at the Rio Film Festival.

"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," said Scahill.

"Dirty Wars" the film, directed by Richard Rowley, traces Scahill's investigations into the Joint Special Operations Command, or JSOC. The movie, which won a prize for cinematography at the Sundance Film Festival, follows Scahill as he hopscotches around the globe, from Afghanistan to Yemen to Somalia, talking to the families of people killed in the U.S. 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 since the first in a series of stories on the NSA spying program appeared in Britain's Guardian newspaper in June. Last week, Brazilian President Dilma Rousseff postponed a scheduled state dinner with Obama after television reports to which Greenwald had contributed revealed that American spy programs had aggressively targeted the Brazilian government and private citizens.

Rousseff railed against the U.S. surveillance during her address to the United Nations General Assembly earlier this week.

Both Scahill and Greenwald applauded Rousseff's reactions to the revelations, but they warned that U.S. spying could be replaced espionage by another government if care isn't taken.

"The really important thing to realize is the desire for surveillance is not a uniquely American attribute," said Greenwald. "America has just devoted way more money and way more resources than anyone else to spying on the world.