By David Alexander

WASHINGTON (Reuters) - The U.S. Army is sending a field artillery battalion of about 400 soldiers and multiple-launch rocket systems to South Korea, a move the Pentagon said on Friday was part of a reorganization of the service and unrelated to any tensions on the peninsula.

Army Colonel Steve Warren, a Pentagon spokesman, said a battalion from the 20th Field Artillery would deploy from Fort Hood, Texas, in June for a nine-month rotation at Camp Casey in South Korea as part of the 210th Field Artillery Brigade.

The unit will leave its equipment in South Korea at the conclusion of the deployment so other battalions can use it as they rotate in on similar deployments, he said. The rotational deployments represent a slight increase in the 28,500 U.S. troops stationed in South Korea, defense officials said.

The multiple-launch rocket system can fulfill a variety of missions but is particularly effective at countering fire from enemy artillery batteries. Officials said they thought the field artillery brigade in South Korea was the only one permanently stationed outside the United States.

"The addition of this battalion is part of an Army-wide reorganization that will raise the number of MLRS (multiple-launch rocket system) battalions in all field artillery brigades from two to three," Warren said.

Lieutenant Colonel Don Peters, an Army spokesman, said that as part of the restructuring, the Army has eliminated some of its field artillery brigade headquarters units. To retain overall combat power, it is spreading the multiple-launch rocket battalions among the remaining artillery brigades.

Peters said the total service reorganization, which results from budget cuts and the winding down of the Afghanistan war, will reduce the number of active duty soldiers to 490,000 from 570,000.

"This was a massive reorganization of the Army back in 2013. Just about every type of unit we have was touched in some form or fashion when we did this," he said.

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As budget cuts continue, the Army is expected to further reduce its active duty force to about 450,000, and possibly go as low as 420,000.

Peters said the Army is examining what further consolidation it would have to undertake to reduce the size of the force to 450,000.

(Reporting by David Alexander; Editing by Peter Galloway)