TOKYO, AUG. 28 -- Japan should bolster its sea-based defensive capabilities immediately to counter a growing threat from the Soviet Union's military buildup in Asia, the Japanese Defense Agency said in a report released today.

The key to that effort would be purchase of early-warning aircraft, advanced radar systems, air tankers for midair refueling and interceptor fighters, the agency said in its annual white paper on the state of Japanese military preparedness.

A budget request for next year, released by the Defense Ministry yesterday, included funding for a destroyer equipped with the U.S.-made Aegis radar defense system.

The budget would also fund a study of the installation of a beyond-the-horizon radar system on Iwo Jima. This would detect aircraft or missile targets thousands of miles away.

The Defense Agency requests are likely to be controversial within Japan and among its Asian neighbors, some of which have expressed concern about a resurgence of Japanese militarism.

Beyond-the-horizon radar and early-warning aircraft might be seen in the region as a sign of Japan's desire to become more of a regional power than it has been since World War II.

The United States has urged Japan to be more active in defending the sea lanes around Japan, particularly in light of the Soviet buildup in Asia.

The United States strongly supported the Japanese decison this year to breach a decade-old limit that held defense spending to less than 1 percent of gross national product. Spending this fiscal year is estimated at 1.004 percent of GNP.

The defense white paper dismissed charges of renewed militarism, saying that there is firm civilian control over defense spending. In addition, it said the increased threat from the Soviets is real. As evidence, it cited the placement of about 170 SS20 nuclear missiles in Soviet Asia (to be removed in the event of a Soviet-U.S, missile agreement), deployment of new Backfire bombers capable of carrying long-range cruise missiles and growth in air and naval forces in the Far East.

The report said there has been a sharp increase in military cooperation between the Soviets and North Korea. It also noted that Soviet planes occasionally appear to conduct mock raids against Japanese radar sites. As a result, it said, Japan's entire air defense system must be modernized.