North Korea is expected to begin fueling a long-range rocket soon, the final step toward its launch set for as early as this week, a South Korean official said Monday.

"North Korea has only fueling remaining after completing the installation of three stages of boosters on a launch pad," the official said on condition of anonymity. "Fueling is expected to begin soon, considering it usually takes two to three days."

Pyongyang says the Unha-3 rocket will blast off between April

12-13 to put what it claims is a satellite into orbit. South Korea, the United States and other regional powers view the claim as a pretext to disguise a banned missile test.

In an attempt to bolster its case, the North has invited foreign journalists and experts to observe the launch. Reporters from more than 20 media firms have arrived in the communist nation, according to state media.

Foreign journalists and space experts have toured the launching station in the country's northwestern area of Cholsan, Pyongyang's official Korean Central News Agency reported Monday.

North Korea told them Sunday that the satellite will gather necessary information on the distribution of forestry resources in the country, severity of natural disasters, crop estimates, weather forecasts and survey of natural resources during polar orbit.

"Seeing the satellite and carrier rocket ... they had deep understanding of the fact that the satellite launch is aimed at space development and use for peaceful purposes from A to Z," the Korean Central News Agency said in an English-language dispatch.

The dispatch also claimed the foreign journalists and space experts "admired to learn that the North's space science and technology has entered the practical stage of satellite."

It also said the satellite launch "is an inspiring deed and an event of historic significance of the nation."

The rocket launch is timed to celebrate the April 15 centennial of the birth of North Korea's late founder Kim Il-sung, the grandfather of current leader Kim Jong-un.

Weather is expected to be a key factor in North Korea's decision on when to launch the rocket. Considering that the regime has said it will accept foreign journalists until Friday, a launch could take place on Saturday or Sunday.

Further adding to tensions, North Korea is also believed to be readying for a nuclear test.

Satellite images show the communist nation digging a new tunnel underground in the Punggye-ri nuclear test site in the country's northeast, where it conducted two previous nuclear tests, first in

2006 and then in 2009, according to South Korean officials.

The construction is believed to be in its final stage, the official said.

Commercial satellite imagery showed piles of earth and sand at the entrance of a tunnel in the Punggye-ri site. The soil is believed to have been brought to the site to plug the tunnel, one of final steps before carrying out a nuclear test blast.

South Korean officials said they cannot rule out the possibility of North Korea setting off a nuclear device built with highly enriched uranium this time, unlike two previous tests where plutonium was used.

The North's nuclear and missile programs have long been a regional security concern. The country is believed to have advanced ballistic missile technology, though it is still not clear whether it has mastered the technology to put a nuclear warhead on a missile. (Yonhap News)