SEOUL — U.S. Forces Korea eventually will allow half of the 28,500 troops stationed in South Korea to serve here with their families, a dramatic change in an area once considered too dangerous for families because of the threat from an unstable North Korea.

As a first step toward what the military calls “tour normalization,” USFK roughly will double the number of command-sponsored slots in the next year — to 4,350 in Seoul, Osan, Pyeongtaek, Daegu and Chinhae.

As many as 2,100 troops in Area I, just south of the heavily-guarded border between North Korea and South Korea, also will get two-year command sponsorships, U.S. Forces Korea commander Gen. Walter Sharp said Wednesday.

“I think it adds deterrence value for North Korea,” Sharp said. “It says: ‘OK, we’re comfortable enough in our capabilities, we’re comfortable that we’re bringing families over here, and you better not miscalculate.” About the same number of troops in Area I, officially called U.S. Army Garrison-Red Cloud, have already brought their families to South Korea, but haven’t received family housing or cost-of-living allowances because they aren’t command sponsored.

Dr. David Chu, undersecretary of defense for personnel and readiness, signed a memorandum on Dec. 1 that approves USFK’s request for the shift, and authorizes changes to the Joint Federal Travel Regulation, which sets tour lengths for U.S. military installations around the world.

Most of those 28,500 U.S. troops in South Korea serve on one-year unaccompanied tours.

Under the new policy, troops serving in accompanied billets will stay for three years in all areas except Area 1. Accompanied tours to Area I will be set at two years.

Unaccompanied tours for most troops will remain at one year, but some “key personnel” will serve 2-year unaccompanied tours, USFK said.

Services may begin extending tour lengths now but are required to submit plans for implementation to Chu by March 1 USFK spokeswoman Col. Jane Crichton said Wednesday.

The move comes after two USFK commanders — Sharp and his predecessor, retired Gen. B.B. Bell — pushed Congress, the Department of Defense and South Korea to approve extended tour lengths. Longer accompanied tours are standard in Japan, Europe and at many U.S. bases in the rest of the world.

Sharp said there was little resistance, but cited many questions among Defense Department officials — “a lot of: ‘How are you going to do that? Do you really have the infrastructure to do that?’ ”

Bell, who retired in June, began lobbying for normalized tours about three years ago, saying the yearlong unaccompanied tours were outdated and that South Korea was modern and safe enough for families. Accompanied tours also would reduce stress on servicemembers who had to leave their families behind, he said.

Sharp said having longer tours “greatly increases” USFK’s military capabilities, and will let him have a more cohesive staff in place as he approaches the critical 2012 transfer of military operational control on the peninsula to South Korea’s military leaders.

“I don’t have to train new people every year,” Sharp said.

Sharp said he didn’t know how long it would take to reach full normalization, but Bell predicted in May that it could take 10 to 15 years.

Crichton said the changes to tour lengths in South Korea will affect worldwide troop rotations, and could take awhile to implement. Services with a small presence in South Korea, like the Marines, may be able to increase tour lengths and command sponsorships faster than those with larger numbers, like the Army, she said.

Sharp said the new policy will affect mostly incoming servicemembers, but those already stationed here can apply for the extended accompanied tours.

He said the pace of normalization depends on how quickly the United States and South Korea pay for the facilities — particularly schools — needed to house and support families. The two countries are in discussions to determine how much each will pay toward the cost of stationing U.S. troops here.

South Korea now pays about 41 percent under a two-year agreement between the two countries. Before negotiations began this year, U.S. officials called for South Korea to increase its support to 50 percent and to have a longer agreement.

Sharp said the longer tours also mean that more middle-grade officers and noncommissioned officers — ranks in high demand in Iraq and Afghanistan — could be stationed in South Korea.

Under the old system, an NCO or middle-grade officer who deploys to the Middle East probably would not be stationed in South Korea immediately before or after deployment, because troops can’t be forced to serve back-to-back unaccompanied tours.

“Effectively, he’s not deployable to Iraq or Afghanistan for a three-year period,” Sharp said.

With the new system, the number of troops available for deployment to Iraq or Afghanistan won’t be affected by increasing tour lengths here, he said. After full normalization, South Korea-based units might be deployable to combat zones, the general said. During a news conference Thursday, he said deployed troops’ families may remain in South Korea.

Sharp said he expects the Defense Department to put more importance — and possibly more money — into the U.S. military presence in South Korea after the 2009 Quadrennial Defense Review, held every four years to assess U.S. military strategic direction.

U.S. bases in South Korea are a key point of its presence in the strategic Northeast Asia region.

“Korea’s a very important part of the world for us,” he said. “It will be after 2012.”