As North Korea prepares to launch its controversial rocket this week, recent satellite images show they are also readying for a third nuclear underground test, according to South Korean media quoting anonymous government officials.

The North is reportedly in the final stages of excavating new tunnels at its Punggye-ri site, in Kilju, North Hamgyong province, where they conducted two nuclear tests, in 2006 and 2009. U.S. commercial satellite photos show piles of dirt being moving in from other areas starting last month. An underground nuclear test requires massive amounts of dirt to fill up the tunnels.

Experts in Seoul say the nuclear test preparation is a calculated move in case the U.S. does not deliver the food aid it had promised on February 29. Washington and Pyongyang had  reached a significant deal then to give 240,000 metric tons of nutritional assistance in return for suspension of North Korea's nuclear program and missile tests.

"This is their hidden card to make sure they get that," said Baek Seung-joo, a North Korea specialist at Korea Institute for Defense Analyses.

But North Korea on March 16  announced it will launch a satellite into orbit between April 12 to 16 to commemorate the one hundredth birth year of its late-founder Kim Il Sung. The U.S. then stated that the planned food assistance would be suspended.

The international community has strongly condemned the launch saying that what North Koreans call "a satellite for peaceful purposes" is actually a cover for testing a long-range missile as both require similar technology. Experts say the Unha-3 rocket could be a test for missiles far enough to reach the U.S. but the North is not believed to have the technology to mount a nuclear warhead on it yet.

In a rare move, Pyongyang has invited foreign journalists to the Tongchang-ri missile launch pad to witness the rocket taking off.

"They invited to show the world that this is not a breach of the deal with the U.S. since it is a rocket, not a missile weapon. So if Americans won't give that food aid which they desperately need, they can say you broke the deal, so we are going ahead and testing another nuclear bomb," said Baek.

On Sunday, foreign reporters were taken by train past desolate fields and sleepy farming hamlets to the new launch pad in North Phyongan province, about 35 miles south of the border town of Sinuiju along North Korea's west coast, according to Associated Press. All three stages of the 91-ton rocket, emblazoned with the North Korean flag and "Unha-3," were visibly in position at the towering launch pad, and fueling is to begin soon. North Korean engineers showed reporters  the 220-pound Kwangnyongsong-3 satellite to be mounted on the rocket.

"The late-Kim Jong Il had set a date and built the pad. There's no way his son and new young leader could or would back down on this one," said Baek, who believes that the launch has a military purpose.  Pyongyang seems to have a high level of confidence that it will reach its goal of getting the satellite up at the range of only 310 miles.

This is the fourth attempt by North Korea to put a satellite into orbit on a three-stage long-range rocket following 1998, 2006, and 2009. The North has claimed each time that it was a successful launch through its domestic media but international analyses show that all were failures.

Neighboring South Korea and Japan have stated that their militaries are prepared to shoot down the rocket if any parts fall in their territories. North Korea's Foreign Ministry has threatened that it would consider any attempt to strike the Unha-3 to be a declaration of war.

Japan, eager to test its ballistic missile-defense system, has been mounting its own show of force in the form of Patriot missile batteries and Aegis-equipped ships, carrying Sm-3 missiles. Three Pac-3s have been deployed in downtown Tokyo, and missile interceptors have been deployed to the Okinawa islands, where Aegis ships now patrol the waters.

ABC News' Eunji Kim and Associated Press contributed to this report.