With growing trade and tourism fueling an increase in calls to and from China, American Telephone & Telegraph Co. and a Japanese company yesterday signed an agreement to expand China's telecommunications links with Japan and the United States.

AT&T declined to reveal some key details of the agreement that it and Kokusai Denshin Denwa, Japan's international telecommunications monopoly, signed with China's telecommunications ministry.

But the accord calls for the first digital fiber-optic link to China -- an 800-mile undersea cable that will be able to carry 35,000 calls simultaneously between the Japanese island of Kyushu and the Chinese coast near Shanghai.

The target date for completing it is 1993.

"Demand for calling between the U.S. and China {has been} going up steadily during the last several years and it continues to grow," said AT&T spokesman Herb Linnen.

"There has been interest in travel to China, but the driving force has been business interests ..." he said.

Last year, more than 1 million calls were made from the United States to China, and during the past four years, calling volume to China has been increasing by an average of 50 percent a year, Linnen said.

He declined to reveal the dollar value of that business, calling it proprietary information.

AT&T, its Japanese partner and Chinese officials have not determined how much the project will cost, how it will be financed or how China will participate in the financing, Linnen said.

The fiber-optic cable will be used by the United States, which now uses satellite links for all of its calls to China.

Linnen said AT&T and Kokusai Denshin Denwa are considering selling some of the fiber-optic cable circuits to private firms.

The first phase of the agreement calls for establishing satellite links to the United States and Japan from major office buildings in Beijing and Shanghai by next year, providing high-speed digital voice and data transmission, primarily for business customers.

AT&T first provided phone service to China in 1937, stopped in the 1940s and resumed service in the early 1970s.