Admiral Thomas Fargo, head of the U.S. Pacific Command, and General Leon LaPorte, commander of U.S. forces in South Korea, say North Korea's nuclear programs, ballistic missile production and illicit narcotics-related activities pose a threat to the region and the world.

The possibility of North Korean weapons or nuclear materials falling into the hands of terrorists is, said Admiral Fargo, "perhaps our biggest fear."

"I think our largest concern would be if nuclear material was sold to al-Qaida, clearly. They have the will and the skill obviously to carry out a devastating terrorist attack so that is kind of a nightmare scenario and that is why we feel so strongly about a non-nuclear Korean peninsula," he added.

Admiral Fargo says the U.S. force structure in the Pacific is well-prepared to respond to any threats and ensure diplomacy is backed by a strong military capability.

For his part, General LaPorte says all signs point to Pyongyang continuing to build its nuclear deterrent. As for the future of U.S.-South Korean military cooperation, he said Seoul and Washington are committed to an "enduring" U.S. presence. "Up to this point in time, in the 'Future of the Alliance' studies, there has been no discussion of troop withdrawals. It has really been a discussion of enhancing capabilities, shaping and aligning forces, for many reasons. But it's an increase in commitment to the Republic of Korea by the United States, we're not lessening our commitment at all," he said.

In General LaPorte's view, the impeachment of South Korea's President has had no impact on the strength of the U.S.-South Korean military relationship.

Lawmakers on the House Armed Services Committee expressed frustration with North Korea, saying it appears the situation is worsening.

Curt Weldon, a Republican from Pennsylvania, said, "I'm troubled by what I perceive to be a continuing worsening of the situation with North Korea. And I say that because they continue to build what they call their deterrence, but which we call nuclear weapons. They seem to be more arrogant in their approach with every passing day."

Another lawmaker, Democrat Adam Smith, said Pyongyang appears to be on what he called, an "inexorable march toward nuclear and long-range missile capability."

Other subjects raised included the situation in the Taiwan straits, and the threat posed by terrorist groups in Southeast Asia.

Admiral Fargo said regional and local terrorist groups linked to al-Qaida continue to pose serious threats to "U.S. and friendly interests," but adds that governments in Singapore, Thailand, Malaysia and the Philippines, have successfully thwarted a number of attacks.