Seoul (AFP) - A one-week window opened Sunday for North Korea's launch of a long-range rocket -- widely-condemned by the international community as a disguised ballistic missile test linked to its nuclear weapons programme.

Signs that a launch was imminent were confirmed when Pyongyang on Saturday shortened and brought forward the launch window it had declared to UN agencies.

The rocket, carrying an earth observation satellite, will now blast off between February 7-14 -- instead of the previous schedule of February 8-25.

UN sanctions prohibit North Korea from the use of ballistic missile technology, and the launch will mark a further serious violation of Security Council resolutions, following the North's nuclear test last month.

Pyongyang offered no specific reason for changing the possible launch dates, but a forecast of clear weather was believed to be the most likely factor.

South Korea's Yonhap news agency cited a Seoul defence official as saying the rocket had been erected on the launch pad at the North's Sohae satellite launch centre, and fully fuelled.

The South Korean military has been placed on alert and, together with Japan, has vowed to shoot down the rocket if it strays over its territory.

The new window means any launch would now take place before the February 16 birthday of late leader Kim Jong-Il, the father of current leader Kim Jong-Un.

North Korea often times such events to coincide with important political anniversaries.

- Superbowl launch? -

It also means the rocket could blast off just before -- or even during -- the most-watched US sports event of the year -- the Superbowl, which kicks of at 8:00 am Monday, Pyongyang time.

The North insists its space programme is purely scientific in nature, but the United States and allies, including South Korea, say its rocket launches are aimed at developing an inter-continental ballistic missile (ICBM) capable of striking the US mainland.

They have warned Pyongyang it would pay a heavy price for pushing ahead with the launch, but analysts say the North's timing has been carefully calculated to minimise the repercussions.

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With the international community still struggling to find a united response to the North's January 6 nuclear test, the rocket launch -- while provocative -- is unlikely to substantially up the punitive ante.

North Korea last launched a long-range rocket in December 2012, placing a similar Earth observation satellite in orbit.

Western intelligence experts said that satellite had never functioned properly, and argued that this proved the mission's scientific veneer was a sham.

The flight plan coordinates for the upcoming launch are almost identical to those followed by the three-stage Unha-3 rocket launched in 2012 -- suggesting the same carrier would be used again.

The separated first stage was predicted to fall in the Yellow Sea off the west coast of South Korea, followed by a second stage splashdown in the Philippine Sea.

- International pressure -

Despite Pyongyang’s bellicose claims to the contrary, the North is still seen as being years away from developing a credible ICBM.

Orbital rocket launches, experts say, are relatively straightforward compared to the challenge of mastering the re-entry technology required to deliver a payload as far away as the United States.

The US-led campaign to impose harsh new sanctions on North Korea over its latest nuclear test have faced opposition from the North's main diplomatic protector, China.

On Friday, both US President Barack Obama and South Korean President Park Geun-Hye spoke by phone with Chinese leader Xi Jinping, urging him to back punitive measures against Pyongyang.

While infuriated by North Korea's refusal to curb its nuclear ambitions, China's overriding concern is avoiding a collapse of the regime in Pyongyang and the possibility of a US-allied unified Korea on its border.