The US military's combat readiness will be undercut and the country's national security will be undermined if lawmakers fail to stop automatic spending cuts in 2014, Pentagon chief Chuck Hagel warned Wednesday.

In a bluntly-worded letter to the leaders of the Senate Armed Services Committee, the defense secretary laid out a dire scenario in which the US military would be forced to take "draconian" steps to save money while jeopardizing its technological edge.

The automatic cuts would have "far reaching consequences, including limiting combat power, reducing readiness and undermining the national security interests of the United States," Hagel wrote.

With no action from Congress, the Pentagon will have to absorb $52 billion in cuts in fiscal year 2014.

Slashing defense spending by that amount would mean "the size, readiness and technological superiority of our military will be reduced" and the country's ability to meet national security commitments would be "at much greater risk," Hagel wrote.

The stern warning underscored growing frustration at the Pentagon with Congress, as lawmakers have failed to break an impasse over the federal budget while rejecting the department's efforts to find savings through base closures and other measures.

Under the automatic cuts that began in March, known as sequestration, the Pentagon already has had to contend with $41 billion in reductions through the current fiscal year, which ends on September 30.

As a result, the Defense Department has scaled back training, curtailed maintenance and imposed 11 days of furloughs on much of its civilian work force, with hundreds of thousands of employees forced to take unpaid leave one day a week over the next three months.

If the automatic cuts stay in effect next year, the Pentagon would have to take much more drastic action. Beyond furloughs, it would have to lay off some civilian workers and make deep cuts in other accounts, according to Hagel.

The former senator appealed to lawmakers to back the department's alternative cost-saving proposals, including closing bases, limiting pay increases, speeding up the downsizing of the Army and reforming the military health insurance.

Without supporting these "difficult but necessary measures," Congress will only aggravate the budget pressures squeezing the Pentagon, he wrote.

Even under the automatic budget cuts, the Pentagon's annual budget will still exceed other countries' military spending at more than $500 billion.