President Carter is trying to balance his fiscal 1981 budget by reducing government spending for butter but not guns, Defense Secretary Harold Brown said yesterday.

At a breakfast at the Pentagon for members of the House Armed Services Committee, Brown said the president so far has kept the defense budget off limits as he issues orders for other government agencies to cut.

But the White House so far is resisting Pentagon requests for a spending increase this year and next.

Congressmen who met with him said afterward that Brown refused to disclose how much extra money the military will need in fiscal 1980 and fiscal 1981 to keep ahead of inflation.

Military leaders have drafted several requests for extra money. But they are becoming increasingly pessimistic about getting these requests approved, Pentagon officials said.

For the last several weeks, W. Graham Claytor, deputy secretary of Defense, has been sending back military service requests for extra money for fiscal 1980 with instructions to make compensating budget cuts elsewhere.

"We're being asked to submit phantom supplementals," complained one budget chief, "ones that will not show up as additions in the overall budget."

The Pentagon civilian leadership, sources said, is showing little inclination to approve requests for anything much beyond the rising cost of fuel. Fuel cost alone, it is now estimated, will run more than $3 billion above previous Pentagon assumptions in fiscal 1980 and over $4 billion in fiscal 1981.

In the memos from the military services to Clayton's office over the last several weeks, these amounts have been requested in supplemental fiscal 1980 funds:

Army, $850 million, including funds to step up the recruiting of volunteers who are becoming harder and harder to find.

Navy, about $300 million, mostly for intensified operations in the Indian Ocean.

Air Force, $3.5 billion, with fuel a big part of this.

Pentagon spokesmen yesterday said that the fiscal 1980 request is still "fluid" and would not be submitted to Congress any earlier than next week.

As for fiscal 1981, the budget Carter is trying to balance, the requests for extra money are still in the rough draft stage in the Pentagon. One official described the situation surrounding the Pentagon's 1981 budget as "total chaos."

Carter has promised that the 1981 budget will be about 5 percent higher than the current one after subtracting money lost to inflation, but soaring fuel costs and other inflation make some military officials doubtful that Carter will be willing to add the billions needed to make good on his "real growth" promise.

Inflation has confronted the military services with a Catch-22 situation regarding their purchases of big weapons such as warplanes. To offset the rising cost of a given airplane, largely due to inflation, the Air Force and Navy have been buying fewer of them. But this means spreading the overhead among fewer planes, pushing up the individual price.