An analysis of the latest satellite images of North Korea's nuclear test site appears to show that frantic activities involving a chain of mining carts are underway, amid rumors that the reclusive Asian nation may be planning to conduct a nuclear test, following its failed rocket launch April 13. 

South Korean intelligence, in a report early this month, said that the North was digging a new tunnel at the Punggye-ri nuclear site which was interpreted as a sign of a covert nuclear weapons program.

The latest analysis of the satellite imagery was obtained from a private satellite operator captured between March 8 and April 18. The analysis estimates that 282,500 cubic feet of rubble have been excavated at the site, where the North conducted its failed previous tests, the Associated Press reported.

While it's very clear from looking at these photos that the North has stepped up preparations for a nuclear test over the past few months, it's unclear exactly when the blast will occur, said Joel Wit, editor of the institute's website.

It remains to be seen whether the nuclear device to be tested has already been placed in the chamber that was being dug up.

Speculation is rife that North Korea was nearing completion of preparations to test a nuclear device for the third time after two attempts in 2006 and 2009, an unnamed source with close ties to Pyongyang and Beijing told Reuters.

Observers have said that North Korea may have developed the capabilities to test a nuclear weapon using highly enriched uranium, which has raised concerns in Washington, Seoul and Tokyo.

The source didn't specify whether uranium or a limited supply of plutonium would be used in the test.

Pyongyang drew heavy international criticism when it decided to test a long-range missile April 13, reportedly aimed at boosting its defense to develop an intercontinental ballistic missile capable of striking the US. North's ambitious attempt to launch the Unha-3 rocket failed when it broke apart shortly after takeoff.

US Secretary of Defense Leon Panetta, speaking to reporters on Tuesday during a trip to Brasilia, said he had no specific information on whether North Korea would go ahead with a test, Reuters reported.

But I again would strongly urge them not to engage in any kind of provocation - be it nuclear testing or any other act - that would provide greater instability in a dangerous part of the world, he said.

A senior North Korean military officer recently claimed that Pyongyang was equipped with powerful modern weapons capable of defeating the US. The Korean People's Army is armed with powerful modern weapons... that can defeat the imperialists (US) at a single blow, Vice Marshal Ri Yong Ho told in a meeting attended by party and military officials Wednesday, the Associated Press reported. The chief didn't disclose any further details about the weapons.

Meanwhile, Beijing has shifted its stance regarding Pyongyang's widely panned rocket launch. China's senior-most diplomat showered praise on Kim Jong-Un and promised to strengthen ties with North Korea during a meeting with Kim Yong-il, the Korean Workers' Party director of international affairs, Sunday.

The traditional friendship between China and North Korea is a precious treasure for our two parties, two countries and our peoples, Dai told Kim Yong-il, who visited Beijing. China is willing to work with North Korea to take friendly cooperation to new heights.