By David Alexander

WASHINGTON (Reuters) - The five-year spending plan outlined by the Pentagon earlier this year would cost $123 billion more than the U.S. Defense Department projected and would violate budget limits set by Congress, the Congressional Budget Office said on Wednesday.

The CBO, in a 49-page analysis, said the main driver of growth in defense spending was the cost of operations and support, which accounted for 64 percent of the Pentagon's base budget in 2012. Pentagon plans would lead to significant rises in the cost of healthcare, salaries and operations and maintenance, it said.

Pentagon officials declined to discuss the report. A spokeswoman said officials had not yet seen the document so it would be "premature for us to comment."

The analysis is likely to complicate efforts to resolve the divisive debate over the Pentagon's budget.

Critics say the Defense Department has not yet cut back enough after a decade of budget increases during the wars in Iraq and Afghanistan. They note that previous defense drawdowns have seen spending fall by 20 percent or more, while current Pentagon projections only call for a decline of about 8 percent.

Supporters warn that the United States is still fighting abroad and faces growing challenges elsewhere around the world, making further cuts in defense spending disastrous. They also warn that cuts could jeopardize critical defense industries.

The Pentagon in February proposed a $525 billion budget for the 2013 fiscal year and projected it would need $2.7 trillion to carry out national security activities over the next five years, a planning period known as the Future Years Defense Program.

The five-year spending plan was $259 billion lower than a 2012 estimate, as the Pentagon cut personnel and programs in an effort to reduce its projected budgets by $487 billion over a decade as required by the Budget Control Act passed by Congress last year.

But the CBO estimated the cost of Pentagon programs over the next five years would be $123 billion higher than projected, wiping out nearly half of the savings the Defense Department sought to achieve to reach the spending limits set by Congress.

The budget reductions proposed by the Pentagon would not enable it to achieve the 10-year spending limits set by the Budget Control Act, the CBO said.

"The cost of (the Defense Department's) base budget plans for 2013 through 2021 is $508 billion higher in nominal terms than the funding that would be available to (it) under the Budget Control Act's limits on discretionary appropriations for national defense," the CBO report said.

The report did not address additional across-the-board spending cuts due to go into effect in January. Those reductions would cut another $500 billion in Pentagon spending over the decade, a move Defense Secretary Leon Panetta has said would be disastrous for the military.

(Reporting By David Alexander; Editing by Paul Simao)