South Korea, the U.S. and Japan are desperately trying to get hold of earth samples from near North Korea's nuclear test site since other efforts to assess its recent nuclear test have fallen flat.

Agents from the three countries are apparently hustling on the North Korea-China border to get their hands on a piece of clod from Punggye-ri, North Hamgyong Province, where the regime conducted a third nuclear test two weeks ago.

On Friday, Chinese security forces were still maintaining tight control in the border cities of Yanji and Tumen 10 days after the nuclear test.

"Rumors are rife that South Korean, U.S., and Japanese intelligence agencies have recruited North Korean defectors or ethnic Koreans in China to get earth samples from Punggye-ri," a North Korean source in Yanji said.

The source added security in the border region has been doubled since the nuclear test, with plainclothesmen deployed at bus terminals.

After the previous nuclear test in 2009, it was also widely rumored that a defector brought earth samples from Punggye-ri and handed them over to South Korean and U.S. intelligence agents, the source said.

South Korea, the U.S. and Japan have managed to detect no radioactivity in the atmosphere that would allow them to assess the strength and nature of the North Korean nuclear device, apparently because the test tunnel was tightly sealed.

An intelligence source said, "Earth samples from the area would provide important clues about the nature of its latest nuclear test."

It is unclear whether the North used plutonium or enriched uranium for the bomb. The regime used plutonium for the two prior nuclear tests but boasted this time that it succeeded in conducting a "high-level" test, suggesting to some observers that uranium was used.

But earth samples will be hard to come by. Defectors who fled the North in January said the nuclear test site is surrounded by electric wire fences and a moat 3 m wide and about 2 m deep, so no one can even think of going near it.

Chinese people in the border region said the device must have been more powerful than the previous ones. A 43-year-old resident of Yanji said, "We didn't feel any vibration during the last couple of nuclear tests in 2006 and 2009, but this time the apartment buildings shook." He said that is why Chinese people are angry with North Korea and have in some cases taken to the streets to protest.

A 37-year-old taxi driver in Yanji said the fear is that Mt. Baekdu on the border, a dormant volcano, could have been affected by the blast.

Mt. Baekdu sits about 240 km from Punggye-ri. Radiation from the nuclear test could have contaminated the underground soil and water and could cause a serious environment problem in a decade or two.

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