US bombers overflew the Korean peninsula on Thursday as part of an exercise with Japanese and South Korean warplanes, the US Air Force said, days before President Donald Trump arrives in the region for a trip set to be dominated by the nuclear-armed North.

Tensions are high over Pyongyang's ballistic missile and atomic programmes, which in recent months have seen it test ICBMs and carry out its sixth nuclear blast.

Trump's visit will throw a spotlight on the issue after the US President and North Korean leader Kim Jong-Un traded insults and threats of war.

Flights by supersonic B-1B Lancer bombers in the area always infuriate North Korea, which condemned the drill as “blackmail” early on Friday.

Two B-1B aircraft took off from Andersen Air Force Base in Guam, and were joined west of Japan by Japan Air Self-Defense Force fighters, the US Pacific Air Force said in a statement.

“The Lancers then transited overland to Korea to integrate with Republic of Korea fighters in the Yellow Sea,” the statement read, adding that the aircraft later returned “to their respective home stations.”

The exercise was part of the “continuous bomber presence” mission in the Pacific and “was not in response to any current event,” the statement said.

The operation follows an October 10 “show of force” in which two Lancers staged the first night-time joint aviation exercises with Japan and South Korea.

North Korea in July launched two intercontinental ballistic missiles apparently capable of reaching the US mainland.

It followed up with two missiles that passed over Japan, and its sixth nuclear test, by far its most powerful yet.

Trump has warned of “fire and fury” in response to Pyongyang's threats, and derisively dubbed Kim “Rocket Man”, who responded by calling him a “dotard”.

This week the North's KCNA news agency described the US president as “incurably mentally deranged”.

Missile test?

Trump will on Friday set off for his Asian tour, which will include Japan, South Korea, China, Vietnam and the Philippines.

But his message risks being undermined if the North stages some kind of provocation, such as a missile test.

It could be preparing for another launch, Seoul's spy agency reportedly said on Thursday.

“Active movements of vehicles have been detected at a missile research facility in Pyongyang,” the National Intelligence Service told a closed-door parliamentary audit, according to the Yonhap news agency.

The North habitually condemns B1-B flights as rehearsals for an attack, and KCNA said Friday: “The reality clearly shows that the gangster-like US imperialists are the very one who is aggravating the situation of the Korean peninsula and seeking to ignite a nuclear war.”

Any military strike in Korea could see retaliation and rapid escalation, and much of Seoul ─ a city of 10 million people ─ is within range of the North's artillery.

The South's President Moon Jae-In has insisted that no military action on the peninsula can be taken without Seoul's approval.

In an interview with Yonhap, Trump's National Security Advisor HR McMaster said diplomacy was Washington's “main effort now”.

“The discussion will also be about what more can we do, what more can all countries do to resolve this short of war,” he said.