North Korea's mobile missile launcher has been seen moving near the east coast carrying a ballistic missile launcher, while activity has also been seen at a long-range rocket launchpad on the west coast, Japanese public broadcaster NHK has reported.

International pressure has grown on North Korea to call off a planned rocket launch, seen by some governments as another missile test, since Pyongyang told United Nations agencies this week it would launch what it called an "earth observation satellite".

The NHK report on Thursday said the mobile missile launcher was thought to normally remain stationary in places such as an underground facility.

North Korea fired two mid-range ballistic missiles, which appeared to be Rodong-class missiles, from the mobile launcher off its east coast into the sea towards Japan in March 2014, Thursday's NHK report said.

Japan has put its military on alert to shoot down any rocket that threatens its territory.

"Japan is saying it will - if any parts of this rocket come down in various stages in Japanese territory - shoot them down if necessary," Al Jazeera's Harry Fawcett reported.

South Korea did not confirm the Japanese report about the mobile missile launcher, but defence officials said Pyongyang was preparing long-range missile at its Dongchang-ri launch site in the northwest

The developments on Thursday came a day after UN Secretary-General Ban Ki-moon called on Pyongyang "to refrain from using ballistic missile technology".

"[These reports are] a deeply troubling development. It will further aggravate the profound concerns that the international community already has in the wake of the recent nuclear test," Ban said in a statement, referring to North Korea's claims of a bomb test in early January.

READ MORE: Japan readies to 'destroy' North Korea rocket

Meanwhile, United States Defence Secretary Ash Carter said on Wednesday that the US military was keeping a vigilant eye on North Korea's missile and nuclear programmes, and was continually expanding its defences against a possible missile attack by Pyongyang.

Carter said the US was on track to expand the number of ground-based missile interceptors in Alaska and Hawaii to 44 from 30, but no further interceptor expansion was planned.