U.S. Secretary of Defense Leon Panetta said Thursday that North Korea is apparently planning for another nuclear test but there is no indication of any imminent action.

"We always get intelligence that they continue to make plans for this, but I have not seen, at least intelligence that I've noted, that indicates that it's imminent," the secretary told reporters.

After talks with Panetta in Washington on Wednesday, South Korean Defense Minister Kim Kwan-jin said the secretive and unpredictable communist nation may carry out a third atomic weapons experiment.

"In fact, North Korea has been preparing for this for quite a long time," Kim told a joint press availability with Panetta. "And when the time comes for a political decision, it may in fact resort to this third nuclear test."

North Korea conducted underground nuclear tests in 2006 and 2009.

The U.S. defense chief also expressed concerns over North Korea's long-range missiles.

"We've been dealing with the threat from North Korea for a long time now in terms of their developing intercontinental ballistic missiles that could reach our homeland and could obviously reach other countries in that region," Panetta said. "That's a threat."

Earlier this month, the North's military declared that it has missiles capable of hitting the U.S. mainland.

The threat came in response to the U.S. decision to let South Korea extend its ballistic missile range up to 800 kilometers from the previous 300 km.

The allies have guidelines for missile technology cooperation. (Yonhap News)