US researchers say they have seen further signs from satellite imagery that North Korea is looking to produce more plutonium for nuclear weapons.

The activity seen at the Nyongbyon nuclear complex comes amid a flurry of weapons-related tests by North Korea.

The website 38 North said commercial satellite imagery taken on Monday shows a rail flatcar at a radiochemical laboratory where North Korea separates weapons-grade plutonium from waste from a nuclear reactor.

It said tanks or casks on the flatcar could be used to supply chemicals or haul out waste products. In recent weeks, exhaust plumes seen at the laboratory also suggest that nuclear reprocessing could be in the works.

The report came as South Korean and US officials said the latest North Korean missile launch, meant to celebrate the birthday of the country's founder, had apparently failed.

It was an embarrassing setback in what was reportedly the inaugural test of a powerful new mid-range missile.

The US and South Korean officials provided few details, including the type of missile, but South Korea's Yonhap news agency carried an unsourced report that a Musudan missile, which could one day be capable of reaching US military bases in Asia and the Pacific, exploded in the air a few seconds after lift-off.

A US official said it appeared to be a Musudan missile but no definitive conclusion had been reached.

Despite the failure, Pyongyang has another Musudan loaded on a mobile launcher and is likely to fire it, according to South Korean and US authorities.

US defence secretary Ash Carter, speaking to reporters during a stop on the USS John C Stennis aircraft carrier in the South China Sea, said that while the US deemed the launch to be unsuccessful, it "was nonetheless another provocation by North Korea in a region that doesn't need that kind of behaviour".

The launch comes as the two Koreas trade threats amid Pyongyang's anger over annual South Korean-US military drills that North Korea calls a rehearsal for an invasion. The North has recently fired a slew of missiles and artillery shells into the sea in an apparent protest against the drills.

The surge in belligerent rhetoric and nuclear and missile activity in the North may also be linked to leader Kim Jong Un's preparations for a major ruling party meeting next month that analysts believe he will use to further solidify his autocratic rule.

Some believe that Kim may try to use the country's claims of recent nuclear and missile success as a way to turn domestic focus towards tackling the country's abysmal economy.