North Korea says rockets to US 'inevitable' as US bombers fly off N Korean coast

North Korea said on Saturday firing its rockets at the US mainland 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 program.

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 in a show of force the Pentagon said demonstrated 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."

A combination photo shows US President Donald Trump in New York, US, September 21, 2017 and North Korean leader Kim Jong Un in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang, September 4, 2017. Reuters

On Saturday, the mudslinging continued with Ri calling Trump "a mentally deranged person full of megalomania and complacency" who is trying to turn the United Nations into a "gangsters' nest".

Ri retorted that Trump himself was on a "suicide mission" after the US president said Kim was on such a mission.

"'President Evil' is holding the seat of the US President," Ri said, warning that Pyongyang was ready to defend itself if the United States 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"

North Korean Foreign Minister Ri Yong-ho addresses the 72nd United Nations General Assembly at UN headquarters in New York, US, September 23, 2017. Reuters

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 programs.

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.

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

"We are prepared to use the full range of military capabilities to defend the US homeland and our allies."

North Korea has launched dozens of missiles this year, several flying over Japan, as it accelerates its program aimed at enabling it to target the United States with a nuclear-tipped missile.

Pyongyang conducted its sixth and largest nuclear test on Sept 3 and has threatened to test a hydrogen bomb over the Pacific.

The Pentagon said the B-1B bombers came from Guam and the US Air Force F-15C Eagle fighter escorts came from Okinawa, Japan. It said the operation showed the seriousness with which it took North Korea's "reckless behaviour."

The patrols came after officials and experts said a small earthquake near North Korea's nuclear test site on Saturday was probably not man-made, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.

China's Earthquake Administration said the quake was not a nuclear explosion and had the characteristics of a natural tremor.

The CTBTO, or Comprehensive Test-Ban Treaty Organization, which monitors nuclear tests, and officials of the South Korean meteorological agency also said they believed it was a natural quake.

The earthquake, which South Korea's Meteorological Agency put at magnitude 3.0, was detected 49 km from Kilju in North Hamgyong Province, where North Korea's known Punggye-ri nuclear site is located, the official said.

All of North Korea's six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test registered as a 6.3 magnitude quake.

North Korean leader Kim Jong Un provides guidance with Ri Hong Sop (2nd left) and Hong Sung Mu (right) on a nuclear weapons programme in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang Sept 3, 2017. Reuters

Tensions have continued to rise around the Korean Peninsula since Pyongyang carried out its sixth nuclear test, prompting a new round of UN sanctions.

Trump told the United Nations on Tuesday the United States would "totally destroy" North Korea if it threatened the United States or its allies.

North Korea's nuclear tests to date have all been underground, and experts say an atmospheric test, which would be the first since one by China in 1980, would be proof of the success of its weapons program.

The United States and South Korea are technically still at war with North Korea because the 1950-53 Korean conflict ended with a truce and not a peace treaty.

The North accuses the United States, which has 28,500 troops in South Korea, of planning to invade and regularly threatens to destroy it and its Asian allies.