North Korea is pushing ahead with a planned long-range rocket launch despite international condemnation, satellite images show, as Japan vowed Friday to shoot the projectile down if it poses a threat.

The images taken Thursday show work to prepare the launch pad appears under way, according to the 38 North website (38north.org) which published several images taken by private US firm DigitalGlobe.

The nuclear-armed North insists it will go ahead in mid-April with what it calls the peaceful launch of a scientific satellite from its Tongchang-ri site in the far northwest.

The United States and other nations say the exercise is a disguised long-range missile test, calling the plan a provocative breach of UN resolutions and of a US-North Korean deal reached last month.

The website, a project of the US-Korea Institute at the Johns Hopkins School of Advanced International Studies, said the detailed images show the mobile launch pad on tracks next to a gantry tower.

It said a crane appeared to have been loading equipment onto the gantry, and numerous small objects and people could be sighted on the pad.

A work crew seemed to be cutting away brush, possibly to prevent the spread of any fire started by the launch.

At the two largest propellant storage buildings to the right of the pad, containing tanks to supply the Unha-3 rocket's first stage, trucks could be seen delivering fuel and oxidiser, it said.

38 North said preparation "seems to be progressing on schedule" and the next step would be moving the first stage to the pad, probably on March 30 or 31.

This would be followed by the second stage a day or two later, with the third stage and payload likely following by April 2 or 3.

"Unless some major setback occurs, the North Koreans will be able to launch during the declared launch window starting April 12," it said.

The North has said it will launch the satellite some time in the morning between April 12-16 to mark the 100th anniversary of the birth of founding president Kim Il-Sung.

It says the satellite will estimate crop yields and collect weather data, among other tasks, and rejects strong criticism from leaders including US President Barack Obama.

Pyongyang said the first stage would fall about 140 kilometres (87 miles) off South Korea's west coast, and the second stage 190 kilometres east of the northern Philippines.

Japan fears the rocket may pass over its southern island chain of Okinawa. Defence Minister Naoki Tanaka said Friday he had ordered troops to shoot it down if it threatens Japanese territory.

Amid international alarm at the upcoming long-range launch, South Korean newspapers said the North test-fired two short-range missiles off its west coast this week.

The North frequently conducts such short-range tests but their timing sometimes coincides with periods of tension.

Irked as what it sees as bad faith, the US has suspended plans to start sending 240,000 tonnes of food aid to the North. The food was part of a deal under which Pyongyang promised a partial nuclear freeze and a missile test moratorium.

Pyongyang insists a satellite launch is not a missile test, a stance rejected by other countries who say the rocket technology is dual-use.

South Korea, which has icy relations with its neighbour, says the launch aims to test technology which could one day deliver a nuclear warhead.

The North is estimated to have enough plutonium for six to eight weapons, but it is unclear whether it has the technology to create an atomic warhead.

The North fired off long-range missiles in 1998, 2006 and 2009. After the two most recent launches it swiftly followed up with an underground nuclear test, and some analysts see a similar scenario unfolding this time.