The U.S. must be more understanding of mainland China's desire to reunite with Taiwan, the Chinese ambassador to the United States said last night at the Kennedy School of Government.

Han Xu, who has served as ambassador for six years, told a crowd of nearly 500 at the Institute of Politics that U.S. support for Taiwan could be a source of frustration in future Sino-American relations.

Han also attacked the Taiwanese government for blocking the free flow of mail, shipping and trade between the two countries.

"Some Chinese have members of their direct families living in Taiwan. How anybody can deny them the right to see their dear ones, I don't understand," Han said. When the communists under Mao Zedong gained control of China in 1949, the Nationalist forces of Chiang Kai-Shek took refuge on the nearby island of Taiwan.

Han, 62, was originally scheduled to speak on "Economic Reform of China," but chose instead to focus on the current state of American-Chinese relations.

Dwight H. Perkins, Burbank Professor of Political Economy, introduced Han as a Chinese diplomat who played "a central part in the normalization of relations between China and the U.S."

Han praised the recent increase in trade between the U.S. and China. Trade between the two countries rose from $6.1 billion in 1984 to $7.3 billion in 1985. But Han, whose talk was sponsored in part by the Harvard-Radcliffe Chinese Students Association, said low oil prices and tough American textile quotas limit China's potential for growth.

"We understand about America's concern with the trade deficit. Let me set the record straight, though. It's China that has a deficit. We need to export more in order to import more," he said.

Han said there could be no "substantive improvement" in Chinese-Soviet relations until Vietnamese troops withdrew from Kampuchea and Soviet forces left. He said that the Soviets also must remove some of the "massive numbers of forces deployed on the Sino-Soviet border."