WASHINGTON, Jan 22 (Reuters) - The U.S. Army cannot show how it arrived at the $70.2 billion price tag for its plan to add 74,000 soldiers to active duty and reserve ranks, a U.S. watchdog agency said on Tuesday.

The Government Accountability Office, or GAO, said the Army's "Grow-the-Force" initiative, intended to help relieve the strains of war in Iraq and Afghanistan, also appeared to have underestimated some costs and overlooked others.

Among costs omitted from the funding plan is $2.5 billion in health care and educational support assistance associated with increased personnel levels, the GAO said in a Jan. 18 report released on Tuesday.

Defense officials had no immediate comment on the report. But the GAO said the Defense Department was generally in agreement with its recommendations, which called on the Pentagon to provide new oversight data to Congress by March 30 and maintain a transparent audit trail of the program.

The Army disclosed plans a year ago to grow its active duty and reserve ranks from 1,037,000 to 1,112,000 by October of 2013. Defense officials have since announced an accelerated plan that would bring the deadline forward to 2010.

The Army hopes the addition of six active-duty combat brigades and 13 support brigades will revitalize and balance its fighting force, which has been placed under severe strain by nearly five years of war in Iraq.

But the GAO said the expansion's funding plan is not transparent or comprehensive enough to "allow decision makers to understand the full magnitude of the funds needed and weigh competing defense priorities."

Army budget officials told the GAO they had limited time to develop their $70.2 billion estimate before President George W. Bush submitted his budget to Congress last February.

"It is not clear how the Army developed this estimate," the report said. "Army documents do not identify key assumptions, limitations or the steps used to develop the estimates." (Reporting by David Morgan; Editing by David Alexander and Todd Eastham)

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