North Korea has warned the US it faces "merciless" retaliation if it releases a Hollywood comedy about a plot to assassinate leader Kim Jong Un.

The Interview stars Seth Rogen and James Franco as a producer and talk-show host who land an interview with the North Korean dictator and are then asked by the CIA to assassinate him.

A spokesman for North Korea's Foreign Ministry said in state media that if the US government does not block the release of the movie, it would be considered an "act of war".

With no independent press of its own, North Korea often holds foreign governments responsible for the content of their media. Pyongyang regularly warns South Korea to prevent its press from mocking its leadership.