Photos show new activity at N.Korea nuclear site

Examining satellite photos, the US-Korea Institute at Johns Hopkins University detected a rise in traffic at the Punggye-ri site but cautioned that there was not enough evidence to assert that a new test was in the works.

The think tank said that there had been no sign of vehicles or people moving at the site for a day after North Korea carried out its third nuclear test on February 12 but that activity had resumed by February 15.

Writing on the institute's 38 North blog, analysts Jack Liu and Nick Hansen said the change over a few days may indicate that North Korea "took safety precautions to ensure radioactivity levels were sufficiently low before sending personnel back into the area."

North Korea is believed to have tightly sealed the site, making it difficult for the United States and other nations to detect from the air whether Pyongyang used uranium -- which would prove it has a second nuclear method in addition to its plutonium program.

But the analysts found activity in two different parts of the site. They said that if North Korea detonated the bomb in a tunnel in the northern area, "then the southern tunnel would be readily available for a fourth test."

North Korea likely used the northern tunnel area for its previous nuclear test in 2009, but it is not known in which area it carried out its latest explosion.

The analysts said that another reason why activity appeared to increase this month was the melting of snow that fell the day after the nuclear test.

Despite widespread international condemnation, North Korea has taken on a defiant tone since its latest nuclear test, leading to fears that it will conduct another blast or long-range rocket test.