* Obama wants to save $400 billion in defense spending
* Pentagon says Gates unaware of decision until Tuesday
* Gates, due to step down, has warned against deep cuts
WASHINGTON, April 13 (Reuters) - The United States would have to abandon some military missions and trim troop levels if President Barack Obama presses ahead with new proposed defense cuts, the Pentagon said on Wednesday.
Obama announced a goal of saving $400 billion on security spending by 2023 as part of a larger objective of cutting the U.S. budget deficit by $4 trillion. [ID:nN12216395]
U.S. Defense Secretary Robert Gates, a holdover from the Republican Bush administration who is expected to step down later this year, has warned repeatedly in the past against further deep cuts in defense spending.
Minutes after Obama announced his austerity plan, the Pentagon renewed those concerns even as Gates endorsed Obama's commitment to a thorough review before making any cuts.
Obama has pledged that his budget reductions will not compromise national security.
Pentagon Press Secretary Geoff Morrell said Gates "has been clear that further significant defense cuts cannot be accomplished without reducing force structure and military capability."
The Pentagon said Gates was not informed of Obama's decision on budget cuts until Tuesday. Morrell said the issue would not affect the timing of Gates' expected retirement.
The Pentagon said it would conduct a broad review of its missions, capabilities and "America's role in the world," identifying alternatives for Obama's consideration.
"The secretary believes that this process must be about managing risk associated with future threats and national security challenges and identifying missions that the country is willing to have the military forgo," Morrell said.
Obama's announcement of proposed defense cuts immediately set off alarms in Congress, where the Republican chairman of the House of Representatives Armed Services Committee, Howard McKeon, said he had "grave concerns" about such large spending reductions while the U.S. military was involved in Iraq, Afghanistan and Libya.
The Defense Department accounts for roughly 20 percent of U.S. federal spending and roughly half of discretionary, non-mandated spending -- making it an attractive target in Washington for lawmakers seeking spending cuts.
Gates has warned over the past year against deep defense cuts, and tried to get ahead of calls for them by spearheading his own efficiency drive.
Gates in January said the United States planned to cut $78 billion in defense spending over five years, including a reduction of up to 47,000 troops. Those cuts came on top of $100 billion cost-savings drive that Gates kicked off last year to eliminate waste, cut poorly performing weapons programs and redirect the money to other priorities. [ID:nLDE7060K4]
"My greatest fear is that in economic tough times that people will see the defense budget as the place to solve the nation's deficit problems," Gates said last August.
"As I look around the world and see a more unstable world, more failed and failing states, countries that are investing heavily in their militaries ... I think that would be disastrous."