The United States is planning to press its allies in Western Europe and Japan in coming weeks to step up their defense preparedness in the wake of the Soviet invasion of Afghanistan.

These plans are described by some administration officials as part of a broader effort now taking shape to get tougher with the allies in many areas where Americans are already making sacrifices, such as defense spending, restrictions on trade with the Soviets and the U.S.-led Olympic boycott.

The opening shot in this campaign, officials say, was the speech on Wednesday by White House National Security Affairs Advisor Zbigniew Brzezinski warning the allies to take more "tangible actions" and that rhetoric and passivity in the wake of the Soviet attack was not enough.

Defense planning now going on, officials say, is aimed at developing specific U.S. recommendations about what the long-term response of the North Atlantic military alliance should be to the changed global situation set off by the Kremlin's attack.

These views will be aired at a round of preliminary NATO meetings next month prior to the major semiannual ministers' meeting in May, the first since the Afghan crisis erupted.

They will also provide guidelines for a likely U.S. effort to press Japan to take greater self-defense measures during a visit to this country this spring by the Japanese prime minister.

Officials suggest there will be no centerpiece within the U.S. proposals to the allies, but rather a mixture of specific undertakings that allow for differing political and economic situations in the alliance.

Two important recommendations, however, are likely to be pressure on the allies to add to their stockpiles of ammunition, which many defense officials view as dangerously low, and to take a new look at their own military manpower reserves.

Among other things, the Soviet invasion late in December has focused on the reality that American forces may have to fight in places other than Central Europe or Asia and that Europeans, especially, may well have to do more on behalf of their own defense than in the past.

The idea, officials say, is not to get the Europeans to double what they planned on doing before Afghanistan, but to take the Soviet threat more seriously now and at least meet alliance-wide pledges of recent years to increase real defense spending by 3 percent and speed up combat readiness.

The so-called "weighted average" within NATO is now about 2.5 percent in terms of increase, and countries that aren't meeting the 3 percent include Denmark, Canada and Italy, with Holland and Belgium borderline cases.

There is special concern about Denmark, which has had virtually no increase in defense and one of the highest living standards in Europe. Neighboring Norway has expressed fears that the Danes will expose southern regions of Norway's flank toward the Soviets.

There is annoyance here over Belgium's continued unwillingness to share in NATO's expensive effort to field a fleet of U.S.-built early warning planes keeping watch over the European frontiers.

And there is fairly widespread agreement that Japan, despite its legal restrictions on the military, is simply carrying too small a burden under today's conditions.

While there is no plan to reduce the more than five U.S. divisions based in Europe, and several other divisions based in the United States remain committed to European defense as initial reinforcements, the situation in southwest Asia could mean that still other U.S.-based divisions targeted for the second wave of reinforcements might in fact have to be used elsewhere in a crisis.

To help plug this potential gap, the Europeans could be pressed to beef up their reserves by either forming more units out of existing manpower or modernizing equipment in the hands of reserves.

West Germany, with some 800,000 ready reserve and home-guard forces, has by far the most formidable reserves in Europe and the only large reasonably well-equipped and trained force that could be mobilized fast and perhaps make a difference on the battlefield.

The Dutch reserves, though much smaller, are also highly rated. But even in these two countries there is need for more modern arms in reserve hands and the room for improvement in Belgium, Denmark, Britain, France, and Italy is considerable, planners say.

The financial costs and political unpopularity in Europe of paying attention to the reserves, however, are major problems, U.S. official concede.

Nevertheless, many strategists believe the reserves could be the key to European defense in depth and avoidance of escalation to nuclear conflict.