Rocket hit on mainland US ‘inevitable’, says N Korea’s Kim

North Korea on Saturday said that firing its rockets at mainland United States was ''inevitable'' after US President Donald Trump called Pyongyang's leader ''rocket man'', in a further escalation of rhetoric between the two countries over the North's nuclear weapons and ballistic missile programme.

North Korean foreign minister Ri Yong-ho's remarks before the United Nations General Assembly came hours after US Air Force B-1B Lancer bombers escorted by fighter jets flew in international airspace over waters east of North Korea, which the Pentagon said showed the range of military options available to Trump.

Ri's speech capped a week of escalating rhetoric between Washington and Pyongyang, with Trump and Kim Jong-un trading insults. Trump called the North Korean leader a ''madman'' on Friday, a day after Kim dubbed him a ''mentally deranged US dotard''.

On Saturday, the insults continued with Ri calling Trump ''a mentally deranged person full of megalomania and complacency'' who is trying to turn the UN into a ''gangsters' nest''. Ri said Trump himself was on a ''suicide mission'' after the US president said Kim was on such a mission, the South China Morning Post reports.

He warned of ''merciless pre-emptive action'' if there was any sign of a US military attack.

'''President Evil' is holding the seat of the US president,'' Ri said, warning that Pyongyang was ready to defend itself if America showed any sign of conducting a ''decapitating operation on our headquarters or military attack against our country''.

''Now we are finally only a few steps away from the final gate of completion of the state nuclear force,'' Ri told the annual gathering of world leaders.

He said sanctions would have no effect on Pyongyang's resolve to develop its nuclear weapons, with the ultimate goal being ''balance of power with the US''.

Trump announced new US sanctions on Thursday that he said allow targeting of companies and institutions that finance and facilitate trade with North Korea.

Earlier this month the UN Security Council unanimously adopted its ninth round of sanctions on Pyongyang to counter its nuclear and ballistic missiles programmes.

The US bombers' flight was the farthest north of the demilitarised zone separating North and South Korea that any US fighter jet or bomber has flown in the 21st century, the Pentagon said.