WASHINGTON - The United States will cut $78 billion in defense spending over five years, including a reduction of up to 47,000 troops, in a politically contentious move that would trim the government's growing budget deficit.

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The cuts, unveiled at a somber Pentagon briefing on Thursday, follow increased White House and congressional scrutiny of military spending, which has doubled in real terms since the Sept 11, 2001 attacks.

They are in addition to a $100 billion cost-savings drive that Defense Secretary Robert Gates had kicked off last year to eliminate waste, cut poorly performing weapons programs and redirect the money to other priorities.

Congress ultimately controls the Defense Department's budget, and lawmakers often block administration efforts to cut military spending that provides jobs in their home districts.

But Gates said the military had to play its part in getting US finances in order.

"As the biggest part of the discretionary federal budget, the Pentagon cannot presume to exempt itself from the scrutiny and pressure faced by the rest of our government," Gates said.

Shares of major defense contractors rose. Lockheed Martin Corp and General Dynamics Corp have programs that would be hit by the reshuffle but were spared from deeper cuts that some investors feared.

Gates, in a half-hour address, said the Pentagon will cope with the belt-tightening by freezing civilian pay, changing economic assumptions and reducing troops starting in 2015, among other items.

That will allow defense spending to keep growing modestly through 2014 before leveling off in 2015 and 2016, Gates said.

He said calls from some in Congress for deeper cuts would be "risky at best and potentially calamitous," citing global tensions that require a strong, modern US military.

Mitch McConnell, the top ranking Republican in the Senate, said on Thursday he believed no US government department was off-limits from belt-tightening.

Other Republicans offered a swift rebuke of the plans, in a sign that the proposed cuts may not be realized despite growing pressure to rein in US government spending.

"I'm not happy," House Armed Services Committee Chairman Howard McKeon told reporters. "This is a dramatic shift for a nation at war and a dangerous signal from the Commander in Chief."