WASHINGTON — Under orders to cut the Pentagon budget by more than $450 billion over the next decade, Defense Secretary Leon E. Panetta is considering reductions in spending categories once thought sacrosanct, especially in medical and retirement benefits, as well as further shrinking the number of troops and reducing new weapons purchases.

Mr. Panetta, a former White House budget chief, acknowledged in an interview that he faced deep political pressures as he weighed cuts to Pentagon spending, which has doubled to $700 billion a year since the terrorist attacks of Sept. 11, 2001. He said that meeting deficit-reduction targets might require another round of base closings, which could be highly contentious as members of Congress routinely fight to protect military deployments and jobs in their communities.

Among other steps, Mr. Panetta said, Pentagon strategists were looking at additional cuts in the nuclear arsenal, with an eye toward determining how many warheads the military needed to deter attacks.

Mr. Panetta also held out the possibility of cutting the number of American troops based in Europe, with the United States compensating for any withdrawal by helping NATO allies improve their militaries. That effort would free up money so the United States could maintain or increase its forces in Asia, a high priority for the Obama administration, and sustain a presence in the Persian Gulf after the withdrawal from Iraq this year, he said.