In private meetings that could have a major impact on U.S.-Japanese military relations, Defense Secretary Caspar W. Weinberger told Japan's top leaders that it is hard to maintain U.S. support for the defense of Japan because of a perception that Tokyo is not carrying its fair share of the financial and military load.

Weinberger ended a three-day visit here today with three hours of meetings with Prime Minister Zenko Suzuki and Defense Minister Soichiro Ito. Judging from an account of those meetings provided by a senior U.S. official and from others, they were marked by considerable candor and a sense of urgency on the part of the United States.

Explaining why he thinks that Japan needs to increase its preparedness, Weinberger is said to have mentioned the calls in Congress for various actions to be taken by Japan, the links that some now make between trade problems and defense, and even calls to bring home American troops.

Weinberger, the officials said, registered "great opposition" to all such suggestions or linkage. But the fact that he mentioned them directly suggests that the discussions now are down to basic issues.

Referring to the battle at home to build U.S. defense strength, and the situation in Japan, the official said, "The question of timing did come up and we said we didn't know how much time we had . . . And that's why we regard this as extremely urgent." That was why, Weinberger said, President Reagan had put such a high priority on it.

Weinberger went on to say, according to the official, that against the backdrop of an American military buildup, the United States could fulfill its defense role in Asia far more effectively and with greater value to Japan if the Japanese would play a greater defense role in the Northwestern Pacific.

"That is unquestionably the fact," the official said. "But you don't help that problem by threatening to bring our forces home if some fixed objective isn't achieved, as has been threatened in Congress. We emphasized that, much as we disapprove of these isolationist attitudes sometimes expressed, it was important to understand that they are being expressed."

Weinberger has said publicly that he is pleased with Japan's 7.8 percent increase in its defense budget this year. But he also has said "substantially greater" increases will be required for several years if Japan is to fulfill Suzuki's assessment that his country's self-defense forces should be able, under their constitution, to protect their homeland and the air and sea lanes out to 1,000 miles. While Suzuki made this pledge, Japanese reports here this morning suggest the Japanese interpret this statement in a much narrower way than do the Americans.

In today's discussion, the senior official, who asked not to be identified, stressed that Weinberger did not make any specific budgetary requests of Japan or demand that a certain share of gross national product be devoted to defense.

The Japanese, according to the official, agreed that they face a threat from the growing Soviet presence in the region and the Soviet occupation of disputed islands just to the north of Japan. But Suzuki is understood to have offered no assurances on the size of future defense budget increases.

The official said the Japanese have not finished cost estimates for a new five-year defense plan but that this will be discussed at a meeting in June in Hawaii.

Weinberger is in the midst of a delicate, but extremely important, effort to tap the extraordinary potential of Japan as a contributor to Asian defense. The Japanese constitution limits Japan only to self-defense rather than offensive forces and there remains considerable hostility in some quarters here to any resurgence of the military.

However, as the increase in the current budget shows, there is a wider recognition now that defense is important and acceptable.

The U.S. mission here essentially is trying to encourage the trend to more defense without breaking that consensus and to convince the Japanese that mutual defense can be made more effective by a combination of improved U.S. and Japanese forces.

While detailed military plans are not discussed openly, the United States would like Japan ultimately to take responsibility for its own defense against air attacks and to be able to protect a large percentage of its own shipping in sea lanes that extend south toward the Philippines and Indonesia and east toward Guam. In addition, some want Japan to help bottle up the huge Soviet fleet based in Vladivostok in the Sea of Japan. That would mean being able to block the three key straits that lead out of that body of water, or at least to harass Soviet warships trying to get through to the U.S. Seventh fleet.

The Japanese self defense force today is good, but small and considerably undersupplied in ammunition and other equipment needed for sustaining a battle. There are 13 army divisions, but the keys to the self defense force are the navy, whose 50 destoyers are more than the U.S. Navy has in the region, and the air force with some 400 planes.

Tokyo also is building 100 new F15 fighters and buying 45 P3 patrol planes over the next several years. These planes roughly have the 1,000-mile reach the United States wants Japan to focus on, but many U.S. planners think Japan will need much more to do the job right. Japan spends less than 1 percent of its GNP on defense.

The Tokyo government, however, is quite conscious of the need for broad political consensus on such controversial moves.