Nissan Motor Co., Japan's No. 2 automaker, will reduce car exports to the United States to ease pressure from mounting inventories, company officials said Tuesday.

They wouldn't specify the scale of the planned reduction. But Japanese press reports, which Nissan officials called speculative, said the company will cut shipments of passenger cars by about 30 percent to 200,000 in the April-September period from 283,000 a year earlier.At the same time, an official of the Japan Automobile Manufacturers Association said the official figures for Japan's car exports to the United States in fiscal '87 would be announced late this month, and the volume would be far below the 2.3 million vehicles sent to America the previous year.

The official, who spoke on condition of anonymity, blamed the shortfall on the higher yen and the slower-than-expected increase in the U.S. car market.

According to unofficial industry figures, Japanese car exports to the United States fell about 86,000 units short of the 2.3-million-unit export limit in the year ended March 31. It was the first shortfall since the restraints were imposed by the Japanese government in 1980.

Japan has decided it would continue the ceiling at 2.3 million units a year for the new year beginning April 1.

Meanwhile, Nissan dealers in the United States are flooded with unsold cars, an auto-industry analyst in Tokyo said.

According to U.S. industry estimates, Nissan had about 155 days' worth of inventories in the United States at the beginning of March, well above the 70- day average for Japanese carmakers as a whole.

Nissan attributes its sales slump in the United States to complete model changes planned later this year for some of its popular cars, such as 200SX coupes and Maxima sedans. Consumers tend to refrain from buying cars that are slated for restyling and other major changes, Nissan officials explained.

Industry watchers said Nissan's newly styled models in the domestic market are proving popular among consumers. However, because of the scale of the company, which sells about 2.7 million units of motor vehicles worldwide annually, it takes about four years to carry out a full, company-wide model changeover.

Nissan began restraining U.S.-bound car shipments late last year as a result of a sales slowdown following price hikes forced under the yen's prolonged strength.

Japanese car makers have raised their export prices to the United States by more than 20 percent on the average over the past 24 months to make up for losses stemming from the dollar's depreciation.