Donald Trump has appeared to threaten regime change against North Korea, tweeting to say if the country's foreign minister echoed the thoughts of its leader they "won't be around much longer."

North Korea's foreign minister, Ri Yong-ho, had earlier said targeting the United States mainland with its rockets was inevitable after "Mr Evil President" Donald Trump called Kim Jong-un "rocket man."

Afterwards, Mr Trump took to twitter to say: "Just heard Foreign Minister of North Korea speak at UN If he echoes thoughts of Little Rocket Man, they won't be around much longer!"

Mr Ri had called the US President "a mentally deranged person full of megalomania and complacency" who was trying to turn the UN into a "gangsters' nest."

The mudslinging came as US bombers and fighter escorts flew to the farthest point north of the border between North and South Korea by any such American aircraft had this century.

The Pentagon said the mission, in international airspace, showed how seriously Mr Trump takes the North's "reckless behaviour."

"This mission is a demonstration of US resolve and a clear message that the President has many military options to defeat any threat," Defence Department spokeswoman Dana White said.

"North Korea's weapons program is a grave threat to the Asia-Pacific region and the entire international community. We are prepared to use the full range of military capabilities to defend the US homeland and our allies," she added.

North Korean foreign minister: Trump is on a suicide mission

North Korea has said it intends to build a missile capable of striking all parts of the US with a nuclear bomb.

Mr Trump has said he won't allow it, although the US so far has refrained from using military force to impede the North's progress.

Last week, the US President announced more economic sanctions against the impoverished and isolated country, targeting foreign companies which deal with the North.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

In a speech at the United Nations last week, Mr Trump had issued the warning of potential obliteration and mocked the North's young autocrat as a "Rocket Man" on a "suicide mission."

His executive order expanded the Treasury Department's ability to target anyone conducting significant trade in goods, services or technology with North Korea, and to ban them from interacting with the US financial system.

Mr Trump also said China was imposing major banking sanctions, too, but there was no immediate confirmation from the North's most important trading partner.