UPDATE 1-US defense chief warns of "crisis" over budget

* Pentagon risks $23 bln shortfall if Congress doesn't act

* Gates defends future belt-tightening, troop cuts distant (Adds comments, background)

By Phil Stewart

WASHINGTON, Jan 27 (Reuters) - U.S. Defense Secretary Robert Gates warned that Congress was causing a "crisis on my doorstep" by failing to approve a key spending bill, inaction that could create a funding shortfall hurting the military.

U.S. lawmakers have not passed the fiscal year 2011 Pentagon appropriations bill and the Pentagon, like the rest of the federal government, is operating under a stopgap measure that keeps funding at 2010 levels.

Gates, in comments made public by the Pentagon on Thursday, appeared to take aim at lawmakers who have voiced opposition to the Obama administration's plans to trim defense spending from 2012 through 2016.

"It's one thing to talk about (fiscal year 2012) and then to express concerns about something that may or may not happen in four or five years, but I have a crisis on my doorstep," Gates said.

"Frankly, that's how you hollow out a military even in wartime. It means ... fewer flying hours, fewer steaming days, cuts in training for home-stationed ground forces, cuts in maintenance and so on."

Gates said congressional inaction would effectively force the Pentagon to operate with $23 billion less than Obama requested.

Arms spending is likely to be one of the main battlegrounds for Obama after he sends his fiscal 2012 budget proposal to Congress the week of Feb. 14. The Defense Department accounts for 19 percent of U.S. federal spending and roughly half of discretionary, non-mandated spending.

(Editing by Doina Chiacu)