WASHINGTON — Defense Secretary Chuck Hagel offered Congress his first formal description of the impact on the Pentagon if across-the-board budget cuts are not lifted by October, writing in a letter on Wednesday that the effectiveness of jet fighter wings, ground combat units and even Special Operations forces would plummet.

If the spending cuts, called the sequester, are not lifted with a deal on taxes and spending, the Pentagon will have to find an additional $52 billion in reductions for its 2014 fiscal year beginning Oct. 1.

“I strongly oppose cuts of that magnitude,” Mr. Hagel wrote. “The size, readiness and technological superiority of our military will be reduced, placing at much greater risk the country’s ability to meet our current national security commitments.”

Faced with such cuts, Mr. Hagel wrote that “two Navy air wings might not be able to achieve full flight hours, and Special Operations units, which are key to counterterrorism activities, would experience declining readiness.”

The Army, he said, would have to cancel training drills, and the Air Force — which Mr. Hagel said had already grounded about one-third of its active-duty combat squadrons — would have to significantly reduce training for more than half of all air combat units.

Mr. Hagel’s eight-page letter was in response to a request on May 2 from Senator Carl Levin, the Michigan Democrat who is chairman of the Armed Services Committee, and Senator James M. Inhofe of Oklahoma, the panel’s ranking Republican.

That request made clear that Senators Levin and Inhofe were concerned that a large segment of the public, and some members of Congress, did not fear the effect of the cuts on military spending — and might even see the reductions as a tool to trim what many of them perceived to be waste in the Pentagon.

But the two senators also wrote that “the identification of these specific reductions will serve both to help Congress and the department prepare for the possibility that we will be unable to avoid another round of sequestration and to show Congress and the public how unpalatable that outcome would be.”

After reviewing Mr. Hagel’s response, Mr. Inhofe said Wednesday that while he would have liked more details, the defense secretary’s letter “makes clear that the devastation of sequester budget cuts will only be amplified beyond what we have experienced thus far.”

Mr. Inhofe warned that “sequestration is leading to the hollowing-out of our military.”

Mr. Hagel said in the letter that while reductions in troop strength would save money in the long run, there would be few immediate savings if a large number of men and women in uniform were fired. That would make them eligible, he said, for separation payments and reimbursements for unpaid leave.

If the sequester cuts remain in effect, Mr. Hagel wrote, a second year of unpaid furloughs will also be required for civilian Defense Department employees.

Money for operations and maintenance would have to be cut by at least 10 percent, Mr. Hagel wrote, and he warned that “would reduce deployable U.S. combat power.”

He said the Pentagon also would be forced to sharply reduce money for new weapons and for the research and tests of the next generation of armaments.

“We would be forced to buy fewer ships, planes, ground vehicles, satellites and other weapons,” Mr. Hagel wrote.

The Pentagon is already seeking ways to reduce its current budget, for the 2013 fiscal year, by about $40 billion to fit the demands of sequester.