Image copyright Sony Pictures Image caption The Interview stars Seth Rogen and James Franco as journalists enlisted to kill Kim Jong-un

A cyber attack on Sony Pictures that forced the cancellation of a major film release is being seen as a serious national security matter, the US says.

A White House spokesman said the US believed the hacking was the work of a "sophisticated actor" - but refused to confirm if North Korea was responsible.

Sony withdrew The Interview, a new comedy film about North Korea's leader, after threats from hackers.

Hackers have already released sensitive information stored on Sony computers.

They later issued a warning to members of the public planning to see The Interview.

Referring to the 11 September 2001 terror attacks, they said "the world will be full of fear" if the film was screened.

Media playback is unsupported on your device Media caption Ex-hacker Marc Maiffret: 'It's easy to break into companies like Sony'

Team America 'pulled'

Three cinemas said replacement screenings of Team America, a film which ridicules the former North Korean leader Kim Jong-il, had also been cancelled.

The Plaza Atlanta and Ohio's Capitol Theatre both tweeted their screenings had been cancelled by the film's makers Paramount Pictures, while the Alamo Drafthouse said the film had been "removed from release". Paramount has declined to comment.

Many cinemas scrapped plans to show The Interview, and Sony then cancelled the release of the film altogether - moves criticised in Hollywood as an attack on the freedom of expression.

At a White House briefing on Thursday, spokesman Josh Earnest said US officials had held daily discussions about the Sony cyber attack and were considering an "appropriate response".

However, he refused to comment on who was responsible, saying he did not wish to pre-empt an investigation by the Department of Justice and the FBI.

Image copyright AFP Image caption North Korea says the film hurts the "dignity of its supreme leadership"

Four possible suspects in the Sony hack

A nation state, most likely North Korea

Supporters of North Korean regime, based in China

Hackers with a money-making motive

Hackers or a lone individual with another motive, such as revenge

Whodunnit? The Mystery of the Sony Pictures Hack

Image copyright AP Image caption Screenings of Team America, which also mocks North Korea, have been cancelled at other cinemas

The Interview, made by Sony Pictures, features James Franco and Seth Rogen as two journalists who are granted an audience with North Korean leader Kim Jong-un.

The CIA then enlists the pair to assassinate him. The film was due to have been released over Christmas.

'Righteous deed'

In November, a cyber attack crippled computers at Sony and led to upcoming films and workers' personal data being leaked online.

The hackers also released salary details and social security numbers for thousands of Sony employees - including celebrities.

Earlier this month, North Korea denied hacking into Sony's computers - but praised the attack itself as a "righteous deed".

An article on North Korea's state-run KCNA news agency, quoting the country's top military body, said suggestions that Pyongyang was behind the attack were "wild rumour".

However, it warned the US that "there are a great number of supporters and sympathisers" of North Korea "all over the world" who may have carried out the attack.

In the article, Sony Pictures was accused of "abetting a terrorist act" and "hurting the dignity of the supreme leadership" of North Korea by producing the movie.