United States trade officials said yesterday an agreement designed to open Japanese markets to American telecommunications equipment companies is off to such a slow start that they are considering not renewing the three-year pact when it expires at the end of the year.

U.S. Trade Representative William E. Brock announced public hearings Oct. 12 and 13 on the agreement and said he is sending a team of trade officials to Tokyo to discuss future American sales to Nippon Telegraph and Telephone Co. (NTT), Japan's government-owned telecommunications monopoly.

"The agreement got off to a very slow start. We've been very disappointed with the actual dollar sales," said Assistant U.S. Trade Representative Douglas Newkirk.

He said the U.S. portion of NTT's $3 billion annual purchases amounted to $15 million in 1981 and $40 million last year.

In contrast, he added, "the Japanese are very successful in our market" with U.S. sales last year of $300 million to $900 million, depending on what products are classified as telecommunications equipment. The Japanese made $20 million in sales to the U.S. government, which was opened to Japan suppliers under the agreement.

The U.S. market is growing due to the court-ordered breakup of the American Telephone & Telegraph System monopoly, which will permit independent phone companies as well as subscribers to buy telecommunications equipment on the open market.

The NTT agreement was aimed at opening the Japanese market to the sophisticated digital switching systems in which American firms excel. "The agreement has taken on special significance as a test of Japan's willingness to open its market to U.S. high technology products," Brock's office said.

Newkirk blamed "bureaucratic inertia" within NTT during the first 18 months of the agreement and a "reluctance" by many American companies to commit the resources to compete in the Japanese market.

The trade official outlined possible U.S. actions if the NTT agreement is not renewed. The options range from continuing to allow Japan free access to the American market to insisting--as some congressmen have suggested--on strict dollar-for-dollar reciprocity in all telecommunications sales between the United States and Japan.