WASHINGTON (Reuters) - The nominee to be the next top U.S. military officer warned on Tuesday that cutting security spending by $800 billion or more as part of deficit reduction measures would be "extraordinarily difficult and very high risk."

Army Gen. Martin Dempsey, President Barack Obama's choice to be the next chairman of the Joint Chiefs of Staff, told a Senate hearing the $14 trillion U.S. debt "is a grave concern" but said he didn't view it as the country's main security threat.

The current chairman, Admiral Mike Mullen, has described the U.S. debt as the single-biggest threat to national security. Dempsey acknowledged an important relationship between U.S. security and the debt but said "that doesn't mean we can neglect the other instruments of national power."

"National security didn't cause the debt crisis nor will it solve it," Dempsey told the committee in a written response to questions.

The hearing on Dempsey's nomination coincides with a deadlock in Washington on finding a way to reduce the $1.4 trillion annual U.S. deficit and $14.3 trillion debt ahead of an August 2 deadline for raising the debt ceiling.

Obama earlier this year asked the Pentagon to find $400 billion in cuts to national security spending over the next 12 years. The Pentagon is reviewing how best to achieve that goal and is expected to present options to Obama.

But lawmakers grappling with the debt have proposed even deeper cuts of $800 billion or even $1 trillion in the coming years.

Asked about the proposals during his hearing, Dempsey said, "Based on the difficulty of achieving the $400 billion cut, I believe $800 (billion) would be extraordinarily difficult and very high risk."

(Reporting by David Alexander and Phil Stewart; editing by Philip Barbara)