North Korea test-fired two short-range missiles off its west coast this week amid international alarm at its planned long-range rocket launch, South Korean newspapers said on Friday.

The North fired what appeared to be two KN-01 ground-to-ship missiles with a range of up to 120 kilometres (75 miles) early Thursday from a missile base near the western port of Nampo, Chosun Ilbo newspaper said.

It quoted an unidentified Seoul government official as saying the launch was apparently aimed at improving the performance of the projectile and was unrelated to the rocket launch scheduled for next month.

The JoongAng Ilbo newspaper carried a similar report but said the launch was on Wednesday. South Korea's defence ministry declined to confirm the reports.

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

To polish military credentials

Analysts say new leader Kim Jong-Un has been trying to burnish his military credentials by conducting short-range missile tests since the death of his father.

The North reportedly test-fired two short-range missiles off its east coast on December 19, the same day it announced the death of leader Kim Jong-Il.

On January 13 South Korean officials said that the North fired three short-range missiles off its east coast in an apparently routine exercise.

Work underway

The country has begun preparing for a rocket launch next month despite international condemnation, satellite images show, as Japan vowed on Friday to shoot down the projectile if it poses a threat.

The images taken on Thursday show that work to prepare the launch pad appears to be 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 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, in 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 and International Studies, said the detailed images show the mobile launch pad on tracks next to the gantry tower.

It said a crane atop the tower was at a 45-degree angle relative to the pad, indicating equipment was being loaded onto the gantry, and numerous small objects and people could be sighted on the pad.

A work crew appeared 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 it will estimate crop yields and collect weather data, among other civilian missions, 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, in international waters between China and the South.

The second stage was expected to splash down 190 kilometres east of the northern Philippines.

'Shoot it down if necessary'

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

South Korea has also said it will shoot down the rocket if it strays over its territory.

The United States has suspended plans to start sending 240,000 tonnes of food aid to the North, which in return agreed last month to 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 currently 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 mastered 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.