North Korea may conduct the second nuclear weapon test to prove that the communist country is indeed capable of producing nuclear weapons, military experts say.

North Korea has tested a nucler device on October 9, 2006. Many countries has doubted that the test was successful.

New satellite imagery shows increased activity around at least two other North Korean sites, a senior U.S. defense official said.

Possible nuclear test preparations include construction of some buildings and other structures, said military officials. Although the purpose of the structures is unclear, officials are concerned because North Korea has left open the possibility that it will conduct another test.

The activity started a few days ago.

Japanese and South Korean officials Tuesday also cited information of possible preparations for a second nuclear test in North Korea.

Russia joined in Wednesday, its foreign ministry urging North Korea to take a "rational decision" that would lead to talks.

On Saturday, the UN Security Council has approved a number of strict sanctions against North Korea that virtually isolate the country.

North Korea proclaimed that U.N. sanctions amount to a declaration of war.