The Pentagon is moving 3,600 U.S. troops from South Korea to Iraq this summer, a shift that highlights the stress on the U.S. Army and promises a significant change in the way the United States helps defend the Korean peninsula.

Defense Department officials announced yesterday their plan to send the 2nd Brigade of the Army's 2nd Infantry Division to Iraq within the next few months to help deal with festering security problems there. The move will deplete U.S. forces in South Korea by nearly 10 percent, the first major shift of resources out of the country in decades.

Pentagon officials stressed yesterday that the move should not be viewed as a sign of waning commitment to protecting Seoul from a North Korean attack, but some members of Congress expressed concern and some experts said it showed the U.S. Army is stretched dangerously thin.

The Pentagon is dipping into forces protecting a volatile region dominated by concern over North Korea's nuclear ambitions, and members of Congress yesterday predicted U.S. troops would be diverted to Iraq from other parts of the globe over the next year. Defense officials had estimated earlier they would drop U.S. forces in Iraq to about 115,000 by summer, but U.S. generals have asked to keep about 138,000 troops there at least through next year.

The 2nd brigade will take with it to Iraq a number of heavy armored vehicles, such as Abrams tanks and Bradley fighting vehicles, part of the U.S. military's effort to beef up force protection as it fights the insurgency. The deployment from Korea will help spell 20,000 U.S. troops whose tours in Iraq were extended earlier this year.

The shift of forces out of South Korea reduces the U.S. presence there to fewer than 34,000 troops, a level that military officials expect to hold steady for at least a year. The U.S. military has maintained a presence in South Korea since the end of the 1950-53 war there that ended without a peace treaty. Senior Defense Department officials said yesterday that they have not decided whether the U.S. troops ultimately will return to Korea or if this will be a permanent drawing down of forces.

Sen. Jack Reed (D-R.I.) questioned the move and announced plans to offer an amendment to the Department of Defense authorization bill this week that would permanently increase the size of the Army by 30,000 soldiers, to a standing force of 512,400. Such a move would cost $3.6 billion annually.

"Our commitment in Iraq has seriously strained the capacity of the Bush administration to deal with North Korea," said Reed, a member of the Senate Armed Services Committee. "The stress on our land forces undermines the nation's ability to pursue a diplomatic strategy backed by unquestioned military power. . . . What signal is the administration sending about our resolve if the U.S. is forced to move troops?"

U.S. military officials and some Korea experts said the movement of troops out of South Korea should not diminish U.S. capabilities to deter an attack there. Deterrence depends more on high-tech armaments than on U.S. ground troops, which officials said are more a symbol than as a realistic defense.

A senior defense official briefing reporters at the Pentagon yesterday said the troop moves would be paired with increased technology and air defenses that could quickly -- and more powerfully -- strike an adversary in the event of an attack in the Asia Pacific region. The official said heavily armored Stryker battalions will rotate into Korea, a Patriot missile brigade will be stationed there, and short-term plans call for increased military capabilities based out of Hawaii, Guam and Japan.

"The United States has the capability to quickly augment air and naval presence in the Asia Pacific region," said Richard Lawless, deputy undersecretary of defense for Asia Pacific policy. "Due to our strengthened posture and the ability to quickly reinforce capabilities throughout the region, we can deploy forces from Korea without assuming additional operational risks."

James Lilley, former ambassador to Korea and China and a former Defense Department official, said he has been expecting the U.S. force in Korea to be drawn down because North Korea's capabilities haven't changed much since the 1960s, other than in the nuclear realm. Concern over those programs was heightened recently after disclosure that U.S. intelligence has concluded that the country has enough plutonium for at least eight nuclear weapons, an increase from an earlier estimate of two.

In a non-nuclear conflict, Lilley said North Korea wouldn't stand a chance against U.S. firepower, even with a smaller military presence.

"I think we're in a pretty good position to pull out, and I think that cutting back is a good idea," Lilley said, adding that soldiers are needed more in Iraq now. "I think it's an important step. It puts the troops where they're needed rather than having them sitting around on a demilitarized zone and conducting maneuvers."

South Korean officials said the government was not surprised by the Bush administration's desire to move troops to Iraq, but added that further talks were needed to ensure that the reshuffling did not raise South Korean security concerns.

South Korea has an army of more than half a million troops, a military force that is gauged as far more modern than the 1.1 million strong North Korean military.

Correspondent Anthony Faiola in Tokyo and special correspondent Joohee Cho in Seoul contributed to this report.