North Korea has stepped up their response to Seth Rogen's Kim Jong-un assassination comedy The Interview by lodging a formal complaint to the United Nations.

Demanding that the comedy be banned, the letter addressed to UN Secretary General Ban Ki-moon says that not doing so would constitute "sponsoring terrorism" and be "an act of war".

"To allow the production and distribution of such a film on the assassination of an incumbent head of a sovereign state should be regarded as the most undisguised sponsoring of terrorism as well as an act of war.

"The United States authorities should take immediate and appropriate actions to ban the production and distribution of the aforementioned film; otherwise, it will be fully responsible for encouraging and sponsoring terrorism, " the message states.

The Interview stars Rogen and James Franco as a television journalist and his producer, who after securing an interview with the North Korean leader are given the task of killing him by the CIA.

North Korea's initial anger following the release of the film's first trailer turned into a veiled threat of war after the foreign ministry released a statement requesting that the film be banned and threatening a "resolute and merciless response" if it is not.

Seth Rogen has already responded to North Korea's comments about the upcoming film.

The Interview is set for release in the UK on 29 October 2014.