Donald Trump's rhetoric makes it sound as though he shares a speech writer with Kim Jong-un, according to a former CIA expert on North Korea.

On Tuesday night Mr Trump warned Pyongyang faced "fire and fury like the world has never seen" if it continued to threaten the US.

It came after leaked intelligence reports showed some in Washington believed the North had gained the ability to miniaturise a nuclear warhead and affix it to an intercontinental ballistic missile (ICBM).

That followed an ICBM test on 28 July which led the UN Security Council unanimously to vote through a new sanctions package on the North.

On BBC Radio Four's Today programme, former CIA analyst Bruce Klingner said the President's choice of words was not "helpful".

He said on Wednesday: "It seems to be language written by the same speech writer as that for Kim Jong-un.

"I don’t think it’s helpful in trying to generate international support for the United States.

"It, in a sense, generates more concern about US intentions and seems to affirm the growing perception that the US may be considering a preventative attack on North Korea."

A military strike on North Korea would have "catastrophic consequences", he said, echoing the analysis of Defence Secretary James Mattis.

During a press conference at his Bedminster, New Jersey, golf resort, Mr Trump had said: "North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen.

"He has been very threatening, beyond a normal statement. And as I said, they will be met with fire, fury and, frankly, power, the likes of which this world has never seen before."

Pyongyang has since claimed it is considering a missile strike on the US Pacific territory of Guam, which is home to two military bases and more than 160,000 people.

In pictures: North Korea military drill Show all 8 1 /8 In pictures: North Korea military drill In pictures: North Korea military drill North Korea's leader Kim Jong Un watches a military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video

Asked whether Mr Trump made him as nervous as Mr Kim, Mr Klingner said: "I think both are showing themselves to be unpredictable rulers.

"The Trump administration seems to have a problem with messaging in that senior officials contradict each other as well as themselves."

However, Mr Kim is not "crazy", he said. He added: "I would disagree with those who say that Kim Jong-un is crazy and that he may wake up some morning and initiate a nuclear attack on the United States.

"The same was said about Kim Jong-il and I don’t think that was correct either. When I was at CIA we did a lengthy personality study of Kim Jong-il and although he, like is son, is a brutal dictator, he’s not irrational."

Mr Klingner, a 20-year agency veteran, now works for the conservative Heritage Foundation think-tank.

He told the BBC the US had been "pulling its punches" in exerting non-military pressure on Pyongyang—despite the UN approval of what Mr Trump called "the single largest economic sanctions package ever on North Korea".

He pointed to the Obama administration's nuclear deal with Iran, of which Mr Trump has been highly critical, as an example of successful diplomatic and economic coercion.

18 years ago Trump said he would bomb North Korea if in this situation

He added: "The US still continues to pull its punches on measures it can take against North Korea as well as Chinese violators of US law.

"I think there’s far more we can do on the pressure option, and that we should exhaust those opportunities, or those possibilities, before initiating a military option."

However, in July Susan Thornton, the acting assistant secretary of the State Department's East Asia bureau, told senators China understood that "we're going to go after Chinese entities if need be" in order to bring Pyongyang to the negotiating table.