Paek Chang-ho, head of the satellite control centre of the Korean Committee of Space Technology, announced the latest preparations, as the US and Japan called on Pyongyang not to go ahead with the launch.
"We are injecting fuel as we speak. It has started," Paek told visiting foreign journalists outside the capital, Pyongyang. "And as for the exact timing of the launch, it will be decided by my superior."
But Hillary Clinton, the US secretary of state, urged North Korea not to go ahead with a planned rocket launch if it wanted a "peaceful, better future" for its people.
"We are consulting closely in capitals and at the United Nations in New York and we will be pursuing appropriate action," Clinton said at a press conference on Tuesday with Koichiro Gemba, Japan's foreign minister, who echoed her remarks.
"If North Korea wants a peaceful, better future for their people, it should not conduct another launch that would be a direct threat to regional security," the chief US diplomat said.
Gemba spoke of US-Japanese co-operation if North Korea goes ahead with the launch of a rocket, which critics say amounts to a disguised missile test by the reclusive nuclear-armed state.
"The United States and Japan would co-operate with each other and the international community, including the Security Council, would take an appropriate measure," Gemba said.
Neither Clinton nor Gemba explained what they meant by "appropriate" action.
South Korea has already threatened to deliver a "firm response" to "provocation" if the North does goes ahead with its planned launch.
Japan has also braced itself for the launch, having put missile batteries on alert to shoot the rocket down if it passes through Japanese airspace.
The Unha-3 rocket is expected to fly past western Japan, raising concerns that a failed launch, or a falling stage of the rocket, could endanger Japanese lives or property.
"They have come pretty far on the question of range, but they still need a lot to resolve in the precision technology needed for [warhead] re-entry and guidance," a South Korean military official who spoke on condition of anonymity, told the Reuters news agency.
The Unha-3 is believed to be the same three-stage liquid-fuelled ballistic missile the North fired in 2009 over Japan, which eventually splashed down after a 3,800km flight, military experts in South Korea said.
The new rocket is believed to have a design range of more than 6,700 kilometres, and can carry a payload of up to 1,000 kg.
The planned launch has already prompted three Asian airlines to make changes to flight paths to avoid the missile.
Philippine Airlines, Japan Airlines (JAL) and All Nippon Airways (ANA) have announced changes to several routes.
Philippine Airlines said in a statement that as the splashdown area of the rocket's second stage was anticipated to be ''just east of Luzon'', all flights passing through the area during the launch period would have their routes adjusted.
These include about a dozen flights between Manila and the US, Canada, Japan and South Korea.
Philippine officials have also declared a no-fly zone and warned ships and fishing boats to avoid the area where rocket debris could fall.
