Twitter has refused to take down a tweet by Donald Trump threatening North Korea, arguing that the statement by the US president was "newsworthy" and in the public interest.

Trump posted a tweet in response to a speech by North Korea's Ri Yong-ho at the United Nations on Sunday, indicating that if the foreign minister "echoed" the thoughts of Kim Jong-un, the two men "won't be around for much longer." It is the latest in a series of hostile statements between the two countries.

The tweet has been interpreted by many, including Ri, as a "clear declaration of war" -- and by some as a violation of Twitter's terms of service.

Twitter's terms of service currently forbid using the platform to make violent threats, either directly or indirectly. Any accounts that engage in such behaviour may be removed or subject to temporary suspension, it says.

Twitter on Monday released a statement saying the company would update its public policy on which tweets should remain in the public domain.

In a series of tweets, the social media platform argued that Trump's tweet did not meet the conditions for removal. However, the company pledged to "consider a number of factors when assessing whether tweets violate our rules" including re-evaluating tweets that are in the "public interest" or deemed "newsworthy."

"This has long been internal policy and we'll soon update our public-facing rules to reflect it. We need to do better on this, and will," the company said.

It also emerged on Monday that an Iranian missile launch mentioned in a tweet by Trump on Sunday had not actually happened.

"Iran just test-fired a Ballistic Missile capable of reaching Israel," the president had tweeted. "They are also working with North Korea. Not much of an agreement we have!"

The footage on which Trump had based his tweet, released by Iran's state-run TV, was in fact believed to be from an unsuccessful launch in January. Two US officials have since confirmed that the video supposedly depicting a recent missile launch was indeed fake.

The Trump tweet has not been deleted.