US President Donald Trump is "mentally deranged" and will "pay dearly" for his threat to destroy North Korea, Kim Jong-Un said Friday, as his foreign minister hinted the regime may explode a hydrogen bomb over the Pacific Ocean.

In a rare personal attack published hours after Washington announced tougher sanctions, the North Korean leader took aim at Trump over his maiden speech to the UN General Assembly in which he branded Kim "Rocket Man" and threatened to "totally destroy North Korea".

Trump "insulted me and my country in front of the eyes of the world and made the most ferocious declaration of a war in history", Kim said, according to the official Korean Central News Agency.

"I will make the man holding the prerogative of the supreme command in the US pay dearly for his speech," which he called "unprecedented rude nonsense".

The dispatch was accompanied by a photo of the North Korean leader sitting behind a desk holding a piece of paper.

"I will surely and definitely tame the mentally deranged US dotard with fire," he said.

On the fringes of the UN General Assembly in New York, North Korean Foreign Minister Ri Yong-ho told reporters Pyongyang might now consider detonating a hydrogen bomb outside its territory.

"I think that it could be an H-bomb test at an unprecedented level perhaps over the Pacific," he said.

However, he added: "It is up to our leader so I do not know well."

- Sanctions -

Trump's new executive order prohibits firms from operating in the United States if they deal with North Korea.

The move was the latest effort to tighten the screws on Pyongyang over its banned weapons programmes, following it sixth nuclear test -- the largest yet -- and the firing of two missiles over Japan in recent weeks.

It also came after the UN Security Council agreed a further set of sanctions on North Korea, aimed at reducing its ability to trade with the outside world.

Analysts say the sanctions show no signs of working, and cautioned that the increasingly ill-tempered and personal exchanges between Washington and Pyongyang did not augur well.

"There are some very dangerous things that could come that move this from theatre to reality. This is the time to be heading them off, not making them feel inevitable," said John Delury of Yonsei University in Seoul.

He added that Kim's rare first-person statement could also have been aimed at North Koreans.

"He is telling his country that the American president at the United Nations said he is going to totally destroy us... but I am not going to let that happen," Delury told AFP.

The statement was published Friday in North Korean newspapers and read out on state television by a star news anchor speaking over a still image of Kim at his desk.

The announcement, which was scheduled to be played repeatedly throughout the rest of the evening, was followed by video footage of a giant missile on a launch pad and North Korean soldiers angrily waving fists and guns in the air.

Ri Yong Suk, manager of the Pyongyang Dental Sanitary Goods Factory, told AFP in the North's capital that she had read the statement in the newspaper.

"I am full of faith that everyone will support the dear leader and safeguard our country," she said.

- 'Still hope for peace' -

Washington has refused to negotiate with Pyongyang, despite appeals to do so from China and Russia, who are both uneasy over Trump's bellicose tone.

However in a meeting with Japanese Prime Minister Shinzo Abe and South Korean President Moon Jae-in, Trump suggested the door to dialogue remained open.

"Why not?" he said, when asked whether there could be talks with Pyongyang.

China wields the most influence on North Korea, providing an economic lifeline. But it also fears the consequences if the regime collapses, such as an exodus of refugees or a US-allied, reunited Korea on its border.

"Negotiation is the only way out and deserves every effort," Chinese Foreign Minister Wang Yi told the General Assembly.

The sentiments were echoed by Russian Foreign Minister Sergei Lavrov, who said "military hysteria is not just an impasse, it's disaster".

North Korean envoy Ri is expected to meet on Saturday with UN Secretary-General Antonio Guterres, who will send out feelers on possible diplomatic talks.

But Chung Sung-Yoon, analyst at the Korea Institute for National Unification, told AFP that the North itself may have shelved the idea of negotiations until it reaches its nuclear goal.

"People say this is all part of its brinkmanship strategy to force the US to come forward for negotiation. But the North is leaving too little room for the US to do so with the latest series of threats and provocations," he said.