Signs that a launch was imminent were confirmed when Pyongyang yesterday 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.
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.
Also Read
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.
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.
North Korea last launched a long-range rocket in December 2012, placing a similar Earth observation satellite in orbit.