The Republican chairman of the Senate Appropriations subcommittee on defense charged yesterday that the Pentagon failed to inform his committee of a plan to send 21,000 more troops to Western Europe and provided no explanation why the increase was needed.

Sen. Ted Stevens of Alaska, who is also the assistant Senate majority leader, told Pentagon officials at a subcommittee hearing yesterday that "since 1975 there has been a slow and steady buildup without informing Congress" of some 58,000 more U.S. troops in Europe, including the 21,000 to be sent this year and next.

It was the second consecutive day that Stevens and other panel members pounded Pentagon officials with hostile questions about the size and cost of American forces in Western Europe and Japan and with threats to reduce those forces because of a belief that allies are not doing enough in their own defense.

It was also the second day that Pentagon officials did not have information available to answer Stevens' questions about the troop increases and costs or to straighten out possible discrepancies in the figures being used by the committee and the Pentagon.

Sen. Mark Andrews (R-N.D.) told assistant secretary of defense Richard Perle at yesterday's hearing that "you've given us no answers whatsoever" with respect to the current 21,000 troop increase. In 17 years of such hearings, the senator claimed, "I've never seen such evasive answers."

Perle had sought to assure the panel that there was "no intent to sneak in" more forces and that eventually the Pentagon and the committee staff would get to the bottom of the numbers problem.

Nevertheless, the hostility and confusion evident at these hearings comes at a time when Congress is certain to make cuts in the defense budget and when sentiment for bringing back some U.S. troops, an idea that the Pentagon strongly opposes, seems to be growing in Congress.

Stevens said Congress has been told to reduce the defense budget by $4 billion to $6 billion and demanded to know on what basis the Pentagon was asking to increase forces in Europe. Because the Pentagon officials did not seem to know exactly what increases, if any, were being made, they essentially did not answer the question.

To emphasize the concern in Congress over the NATO costs issue, Sen. John C. Stennis, the Mississippi Democrat who for many years was chairman of the Senate Armed Services Committe, joined the panel and warned the Pentagon witnesses that Congress wanted and was entitled to a usable estimate of the costs.

Perle had explained that it is difficult and even misleading to try and precisely estimate the costs of the U.S. commitment to NATO, which Stevens claimed has risen to $133 billion, because those forces could serve elsewhere if necessary, such as in the Persian Gulf. He said they would cost a lot even if based in this country.

The debate over the increases in troop levels centers around the overall 58,000 increase since 1975, which Stevens says he learned from Pentagon statistics but which Deputy Secretary of Defense Frank C. Carlucci disputed in Wednesday's hearing. The planned increase of 21,000 troops this year and next in addition to the 337,000 troops already in Europe comes from material supplied to a Senate Armed Services manpower subcommittee on Feb. 26 by undersecretary of defense Fred C. Ikle.

Stevens claimed he was "startled" and "appalled" to discover this since "we have never been told" about it. But neither Perle nor Air Force Gen. William Y. Smith, a deputy commander in Europe, said they could substantiate the numbers used by Ikle.

Ikle had told the other subcommittee that 9,500 more troops would go to Germany to improve combat readiness of ground and air forces and as part of the deployment of new cruise missiles there. Another 3,300 airmen would go to U.S. air wings in Britain. The Navy and Marines in the area would be increased by 5,200 and another 3,000 troops would be spread among other countries.

The Pentagon officials also could not answer questions about why the military was requesting $23 million for a brigade in Germany that was supposed to be deactivated.