Rumsfeld acknowledged it is more difficult than ever to know what is happening inside the hermetically closed regime of North Korea's Kim Jong-Il.

Intensive North Korean use of underground facilities and a switch to fiber optic communications to foil eavesdropping have further complicated efforts to understand North Korea, he said.

"We know repression works. We know you can put so much fear in people you can maintain your regime for decades with fear, and that it is possible to subjugate people semi-permanently," he said.

"We also know nothing is forever in life, and that at a certain point things can happen. We've seen dramatic shifts in countries where they've gone from here to there," he said.

A succession of moves by North Korea over the past year to overtly revive its nuclear weapons program has sharply heightened tensions in the region.

Rumsfeld said the United States was right to pursue a diplomatic approach to the North Korean problem, working with other countries to put pressure on Pyongyang to moderate its behavior.

President George W. Bush has offered Pyongyang written security guarantees if it accepts a complete, verifiable elimination of its weapons of mass destruction and related programs.

But in a speech to US troops earlier in the day at Osan Air Base, where F-16 fighters and A-10 attack planes are at the ready, Rumsfeld made clear that Washington still holds the bleakest of views on the North's Stalinist regime.

He told the airmen they worked "on the border between freedom and slavery, between democracy and communism, between prosperity and poverty -- a divide so great that people in the North, repressed people to be sure, watch their children waste away (and) eat bark as that evil regime spends huge sums on weapons."

They were Rumsfeld's most slashing remarks against North Korea during a weeklong trip to Guam, Japan and South Korea in which the secretary appeared at pains to avoid interfering with efforts to arrange a new round of six-party talks with the North.

The North Korean threat, however, looms over an ambitious US effort to revamp its military posture on the Korean peninsula and in the region.

In theory, the plan is to replace what Rumsfeld says are static Cold War defenses with more mobile forces that make full use of US advances in precision weaponry, long-range strike capabilities and digitized surveillance and communications.

In Seoul, Rumsfeld and South Korean leaders reaffirmed plans to first consolidate the estimated 16,000 ground troops in camps along the Demilitarized Zone and then pull them back in a second phase into two hub bases south of the Han River.

Rumsfeld gave strong assurances everywhere he went that any "adjustment" in US forces -- his euphemism for troop reductions -- would be more than offset by greater capabilities.

"We do not want to inject instability in this peninsula," Rumsfeld said in Osan after noting that Washington has stood by its commitment to defend the peninsula for 50 years.

At the same time, he said Roh was right to say recently that the time has come for South Korea, which has a modern 700,000-strong military, to become more self-reliant.

Whether the South Koreans buy the US argument remains to be seen. The two failed to agree in annual security talks Monday in Seoul on relocating some 7,000 US troops based in Seoul, something Washington has pressed for since

And South Korea will have a large say in the timing of any move from the demilitarized zone to the south because it must acquire the land and build the facilities where those troops will be based.

Rumsfeld, who held talks in Tokyo, also appeared to be looking forward to Japan's playing a progressively larger security role in the region, even as the Pentagon retools its military presence there, particularly in Okinawa.

In Guam, Rumsfeld found only eagerness for an expanded US military presence. A US territory, the island was once a major military outpost during the Vietnam War and appears poised to become a central hub for US air and naval forces in the western Pacific.

"We already have a significant presence, and clearly it is attractive," Rumsfeld told reporters.

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