The US has sent an aircraft carrier, destroyers and other ships carrying 14,000 servicemen to waters near Japan for a war drill that has set North Korea's teeth on edge.

The 10-day exercise - which will see US and Japanese troops performing war games in the waters off Okinawa - comes as the Kim regime continues to pursue its nuclear missile development program.

It comes just days after North Korean Ambassador Ja Song Nam slammed US drills in a letter to UN Secretary-General Antonio Guterres, saying that the current situation around the Korean peninsula was 'the worst ever'.

US warships began war games in the sea near Japan on Thursday, shortly after exercises (pictured) in the western Pacific with Japan and South Korea, drawing North Korea's ire

The drills began on Thursday and include aircraft carrier the USS Ronald Reagan, and the guided-missile destroyers USS Stethem, USS Chafee and USS Mustin, among others.

North Korea, which has stoked regional tensions with nuclear and missile tests in recent months, has repeatedly denounced such military drills as rehearsals for invasion and sometimes conducts its own military manoeuvres in response.

The Kim regime said that situations in the region around North Korea are the 'worst ever' and that the US is risking war by taking part in the annual drills

The annual drill 'is designed to increase the defensive readiness and interoperability of Japanese and American forces through training in air and sea operations,' the US Navy said.

It comes after three US aircraft carriers, including the USS Ronald Reagan, held rare joint drills in the western Pacific over the weekend.

They were joined separately by Japanese navy and South Korean warships in drills that led to Ja's furious letter.

The drills at the weekend - also the first such deployment of three US carriers since 2007 - 'is making it impossible to predict when nuclear war breaks out due to the US nuclear war equipment' taking up a 'strike posture,' the ambassador wrote.

North Korea conducted its sixth nuclear test this year and test-fired a series of advanced missiles, including intercontinental ballistic missiles.

On Wednesday, Trump vowed a global campaign of 'maximum pressure' on North Korea, warning Pyongyang will not subject the world to 'nuclear blackmail'.

Defending an almost two week trip to Asia that was long on pomp but - critics say - short on achievements, Trump said he had successfully galvanised opposition to North Korean proliferation.

As the drill kicked off, Japanese Prime Minister Shinzo Abe met the commander of the US Pacific fleet, Harry Harris, and urged 'close coordination' between Tokyo and Washington to 'enhance deterrence' amid the North Korea threat.