Defense Secretary Caspar W. Weinberger urged Japan today to step up the pace of its military preparedness, emphasizing that while the U.S. commitment to help defend Asia is solid, the region is so vast that more effective Japanese forces are needed for the common defense and overall stability of the region.

In a speech prepared for delivery at the Japan Press Center here outlining administration attitudes toward the defense of Asia, Weinberger stressed that the United States "will remain a Pacific power." He gave his "pledge" that this country will "within its means, do as much as is necessary" to keep the Pacific-Indian Ocean sea lanes open" that carry much of the oil to Asia and Western Europe.

"The American commitment, however, is larger than we would like because of the magnitude of the area to be covered and the aggressive growth of the Soviet challenge," Weinberger said.

Weinberger pointedly recalled that Japanese Prime Minister Zenko Suzuki, during a visit to the United States last May, said that Japan, within the limits of the self-defense provisions of its constitution, could "defend its own territory, the seas and skies around Japan, and its sea lanes to a distance of 1,000 miles" from the homeland.

The Reagan administration has seized upon this to try to nudge Japan into taking greater responsiblity in the Pacific and to ease some of the strain on U.S. naval and air forces now stretched thin in Asia especially since the need for big deployments near the Persian Gulf arose in 1980.

While Japan has improved its defense capabilities, Weinberger said, "our concern lies with the pace of these efforts."

Today he said, Japanese forces "would have difficulty defending Japan" and the prospect of eventually defending air and sea space out to 1,000 miles "will require substantial improvements in military capabilities . . . and increases in defense spending substantially greater than the current annual growth rate."

The Japanese recently increased their defense budget by 7.8 percent, far greater than any other segment of federal spending in Japan. Washington clearly is pleased at this but also believes that Japan must do much more and do it faster. Thus Weinberger's speech is meant to keep the pressure on, but without harsh public rhetoric, and to explain how Japan's Self-Defense Forces are needed to complement a U.S. military buildup.

Specifically, the Pentagon wants Japan to expand significantly its antisubmarine warfare and air defense forces and the supply stockpiles that would be necessary to sustain those forces in battle.

In contrast to about 5.9 percent of gross national product spent by the United States on defense, Japan spends just below 1 percent. That 1 percent ceiling has become an unwritten but accepted restriction of successive governments here.

Because of its huge gross national product, however, even that small percentage makes Japan's defense budget the world's eighth largest. The U.S. administration, sources say, is not focusing publicly on that 1 percent figure, in part because it fuels emotional arguments in this country. Rather, Washington wants Japan to decide on its own to speed up its five-year plan for defense and believes that if Japan meets those goals it would have to go beyond 1 percent.

Weinberger said today Japan has the capability to provide that 1,000-mile self-defense screen "within this decade."

"Of course, no one in the United States wants to see Japan become a military superpower," Weinberger said. But Japanese sea and air defense forces in the northwest Pacific, he said, could complement U.S. forces there and in the southwest Pacific and Indian oceans.

Japanese critical of expanding defense say their country's security is best preserved by maintaining friendly ties with all countries.

Weinberger, in his prepared remarks, also cautioned against the "obvious emotional appeal" of linking Japanese defense efforts to the strain over trade issues.

"We do not believe that action on trade problems resolves our concerns with defense--or vice versa. Clearly we hope that Japan will continue to widen access to its markets by removing barriers which still make meaningful outside participation in the Japanese miracle difficult." Equal access, he said, "rather than an absolutely equal balance of trade, will allow Japan and the U.S. to coexist as friendly economic competitors."