Responding directly for the first time to US President Donald Trumps threat at the UN to destroy nuclear-armed North Korea, its leader Kim Jong-un called Trump a “mentally deranged US dotard” as the regime’s Foreign Minister hinted it may explode a hydrogen bomb over the Pacific Ocean.

In a rare statement, Kim Jong-un said that Trump would “pay dearly” for his threat, a state-media report said on Friday. The leader said he “will consider with seriousness exercising of a corresponding, highest level of hard-line countermeasure in history”, the Korean Central News Agency (KCNA) report said.

“I am now thinking hard about what response he could have expected when he allowed such eccentric words to trip off his tongue.

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.

“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”.

“Action is the best option in treating the dotard who, hard of hearing, is uttering only what he wants to say,” Kim added. “I will surely and definitely tame the mentally deranged US dotard with fire.”

Kim’s use of the term “dotard”, had set the internet alight. While not widely used today, the insult is centuries old, appearing in medieval literature from the ninth century.

Searches for the term spiked in the wake of Kim’s address, according to dictionary Merriam-Webster, which defines the term as referring to “a state or period of senile decay marked by decline of mental poise”.

Referring to Trump’s Tuesdsay speech at the UN General Assembly, Kim said: “A frightened dog barks louder. He is surely a rogue and a gangster fond of playing with fire, rather than a politician.”

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.”

In his speech, Trump called North Korean regime a “band of criminals” and Kim a “Rocket Man” on “a suicide mission”.

“The US has great strength and patience, but if it is forced to defend itself or its allies, we will have no choice but to totally destroy North Korea,” he said.

Japan’s Defence Minister Itsunori Onodera said the country must ready itself for the sudden escalation in tensions and be prepared for a missile launch.

“We cannot deny the possibility it may fly over our country,” Onodera said on Thursday. Japan had been subjected to two North Korean missile test flyovers in recent weeks.