FBI Director James Comey says he's confident that North Korea was behind the cyberattack on Sony.

Comey said Wednesday that threats made against Sony were traced to IP addresses used exclusively by the North Koreans. He made the remarks at a cybersecurity conference at New York's Fordham University.

Comey said there was evidence that North Korea had sought to use proxy servers to conceal the Sony hack. But he says they sometimes "got sloppy" and didn't use the servers.

Comey predicted that North Korea would try to strike again.

Earlier Wednesday, Director of National Intelligence James Clapper said North Korea will continue the attacks against American interests unless the United States "pushes back."

U.S. sanctions 'an important signal'

The nation is believed to have orchestrated the attacks in retaliation for Sony’s production of The Interview. The movie presents an unflattering image of North Korean leader Kim Jong-un and includes a scene of his death.

The Interview

Sony initially pulled The Interview, starring Seth Rogen and James Franco, from theatres. (Columbia Pictures)

The hackers threatened violence against theatres that would screen the movie, and Sony initially decided to pull it from theatres.

The Interview was later released online and in a limited number of theatres. It grossed more than $1 million US in a limited Christmas Day release.

On Monday, Sony CEO Kazuo Hirai made his first public comments on the attack, praising employees and partners for standing up to the hackers' "extortionist efforts."

The United States has imposed sanctions against North Korea since the attacks, targeting top state officials and defence-related organizations.

North Korea has criticized the U.S. for the new sanctions. Comey called the sanctions "an important signal" of how seriously the government will take such attacks.

With files from CBC News and Reuters