Most of America's European allies seem to be moving toward reduced defense expenditures next year that would prevent them from fulfilling their commitments to join the United States in bolstering the NATO alliance.

With West Germany now considering a significant defense cutback, its pledge to assume a greater defense role in Central Europe may become more difficult to honor at the time the United States and other allies are developing a new rapid deployment force for possible use in Southwest Asia.

Only the United States, no matter who wins today's presidential election, oil-rich Norway, and France, which is not a full member of the alliance, are now certain to increase their defense spending next year by at least the target of 3 percent over inflation pledged by the NATO allies for 1979 onward.

This could create serious new diplomatic conflict between the United States and its allies over their growing inability or unwillingness in a harsh economic climate to shoulder their share of the cost of countering the continuing military buildup of the Soviet Union and the Warsaw Pack nations.

Belgium, Denmark and the Netherlands have all announced that they will fall well short of the 3 percent target increase. And both Britain and West Germany are considering budget cuts that would pull their planned increases in defense spending to below 2 percent or less.

Like the Thatcher government here, West Germany's newly reelected coalition government headed by Chancellor Helmut Schmidt reportedly has decided that defense spending cannot be exempted from across-the-board cuts in an austerity government budget for next year because of a gloomy economic outlook and an increasing balance of payments deficit.

West German officials stressed that final decisions have not yet been made.

But a number of sources said the coalition partners, Schmidt's Social Democratic Party and the smaller Free Democratic Party, agreed on the general shape of the new budget, including the defense cuts, in week-long negotiations on a new government program to be presented to parliament late this month.

While military sources complained that the defense budget might not even rise by as much as inflation, as reportedly proposed by Finance Minister Hans Matthofer, others said they expected Defense Minister Hans Apel to win a military spending increase of 1.75 percent above inflation, still considerably below West Germany's 3 percent pledge to NATO. This is expected to delay West German purchases and deployment of some new fighter aircraft, combat tanks and Navy frigates.

It is unclear what effect the switch would have on Bonn's commitment to increase its share of NATO's defense workload. Because West Germany does not want to be involved in military action outside the NATO area, it agreed instead to free the United States of some of its NATO workload in Europe following the Persian Gulf crisis and the Soviet invasion of Afghanistan.

American Ambassador Walter Stoessel met today with Defense Minister Apel to formally propose that Bonn fulfill this pledge by paying for improved housing and other facilities for U.S. troops stationed in West Germany and by taking the responsibility for heavy arms and equipment stockpiled there for use by airlifted U.S. reinforcements if war broke out. No decisions were expected today.

American diplomatic sources said West Germany's overall NATO defense spending pledge was not on the agenda of today's meeting because Bonn's budgetary process had not been completed. However, NATO officials and defense analysts are closely following reports of the proposed cuts in defense spending because West Germany maintains Western Europe's largest military force and is an important influence on smaller NATO nations, particularly Denmark and the Netherlands, whose defense spending also is in doubt.

In Britain, leaks apparently from defense officials of details of the treasury's proposals to make deep cuts in planned defense spending for the next three years have predictably forced the Thatcher government to reconsider the size of the cuts during an already politically painful review of government defense spending that will continue at least through next week.

But Thatcher reportedly is resigned to the fact that defense must bear some of the brunt of the government spending cuts she believes are vital to her austere monetarist economic policies and their acceptance by hard-hit private business in Britain. Defense officials now believe that Britain's military spending will increase by between 2 and 3 percent above inflation this year (instead of an intended 3.5 percent) and possibly around 2 percent after that, although final decisions have not yet been made.

A three-month moratorium on new defense procurement contracts was ended this week by Defense Minister Francis Pym, but his spending remains constrained by a Treasury-imposed ceiling for the rest of the year.

While expressing their strong disappointment about the failure of richer NATO nations to honor their defense spending pledges, NATO analysts noted Britain's particularly dire economic situation and the determination of British defense officials to come as close as possible to the 3 percent target "we trust the British government to try," said one NATO source.

There have so far been no reports of Carter administration pressure on the Thatcher government to meet its NATO spending commitment. But Washington has made known the pressure it has put on richer allies, particularly high-income West Germany and Denmark, most recently in a letter to the West German foreign minister from Secretary of State Edmund Muskie and a letter to the Danish defense minister from Defense Secretary Harold Brown.

Brown's letter to the Danish government, which has so far proposed a "zero increase" above inflation in its defense spending over the next four years, was especially ominous. "Unless Denmark is able, and is seen to be able, to carry out" its pledge to NATO, Brown wrote to Danish Defense Minister Paul Sogaard, "I will find it extremely difficult to justify to Congress, and the American public, commitments to reinforce Denmark" with U.S. troops in time of war.

After negotiations this winter with right-of-center opposition parties that want a bigger rise in Danish defense spending, the left-of-center minority Social Democratic government of Prime Minister Anker Jorgensen is expected by Danish defense officials to settle for a rise in military expenditure of about 1.5 percent above inflation, similar to the Netherlands and Belgium. Military experts expect this to force significant military manpower cuts in Denmark, leave some units well below strength in Belgium and delay modernization of Dutch military equipment.