Sony CEO Kaz Hirai has spoken for the first time about the "vicious and malicious" cyber-attack which struck Sony Pictures late last year.

Speaking at the CES technology trade show in Las Vegas, Hirai also praised Sony employees and partners for standing up to "extortionist efforts" of the hackers who broke into the company's computer systems and leaked thousands of private documents.

The US government has blamed North Korea for the attack, which resulted in Sony film The Interview (with a plot to assassinate Kim Jong-un) being briefly cancelled ahead of its Christmas Day release. Instead of appearing in cinemas, the film was released online on 24 December.

Despite the FBI's insistence that North Korea is responsible for the attack - and sanctions imposed on the country - some members of the cyber-security community are less than convinced, instead believing an insider, possibly a disgruntled former employee, is to blame.

Hirai said Sony Pictures employees were "the victims of one of the most vicious and malicious cyber-attacks we have known in recent history. Freedom of speech and freedom of expression are...lifelines of Sony and out entertainment business."

Hirai went on to thank those who had seen the film.

The Sony CEO also used his time on stage at Las Vegas Convention Centre to highlight his company's efforts to push 4K, or Ultra HD television, into the mainstream; the company also announced a new Walkman music player with a $1,200 (£790) price tag and a partnership with Virgin Atlantic to bring its SmartWatch 3 wearable to airline staff.