President Reagan has told Chinese leaders that U.S. military sales to Taiwan can be expected to decrease as China makes progress in its effort to peacefully reunify with the breakaway island, according to letters made public by U.S. officials today.

The letters, released as Vice President Bush ended his visit to China, spell out publicly for the first time U.S. conditions for meeting the Chinese demand to cut off all military supplies to Taiwan's Nationalist government.

Although Reagan's offer falls short of Peking's insistence on a deadline for the cutoff, it marks a shift from past administration statements that the United States is indefinitely obligated by law to provide Taiwan defensive arms.

Taipei consistently has spurned Communist reunification overtures as "sugar-coated poison," and it is unclear whether Peking will accept Reagan's condition that it make progress toward a peaceful unification before the United States reduces arms sales to Taiwan.

The president's offer, which his letter disclosed was first made privately to Chinese officials seven months ago, is seen as an attempt to defuse the dispute over arms sales to Taiwan by persuading Peking that he is not trying to create "two Chinas."

Reagan, whose longtime support for Taiwan's noncommunist government has sown distrust in Peking, used the letter, written April 5 to Vice Chairman Deng Xiaoping, to state in his most explicit terms that the administration gives primacy to its relations with China.

"There is only one China," he wrote to Deng, China's most powerful official. "We will not permit the unofficial relations between the American people and the people of Taiwan to weaken our commitment to this principle."

Reagan said he recognizes the "significance" of China's reunification overtures, adding that his decision in January to reject Taipei's bid for more advanced aircraft reflects "our appreciation of the new situation created by these developments."

In a separate letter also dated April 5 and addressed to Premier Zhao Ziyang, Reagan said he welcomed Peking's efforts to regain Taiwan by peaceful means and linked future U.S. military sales to Taiwan to the success of the reunification drive.

"We expect that in the context of progress toward a peaceful solution, there would naturally be a decrease in the need for arms by Taiwan," the president said, noting he made similar points in an October meeting in Washington with Foreign Minister Huang Hua.

Peking, which has threatened to downgrade relations with Washington if it continues selling arms to Taiwan, has not publicly responded to the letters.

Bush, who spent two days in Peking conferring with Chinese leaders, said at a news conference before leaving today that progress had been made in resolving the dispute. He said he had "some specific ideas" from officials in Peking to bring back to Washington.

"I believe the matter can be resolved and will be," Bush said, refusing to elaborate on specific proposals made during his visit.

Bush said talks would continue on the ambassadorial level, adding that discussions will proceed without a deadline. Washington has agreed to suspend arms sales to Taiwan while engaging in negotiations with China on the issue.

The official New China News Agency reported Bush's optimistic view of the talks but offered no analysis. Last night, Vice Premier Wan Li said at a banquet hosted by the vice president that his visit had been "useful" and proved the need to solve the issue.

When Washington shifted its recognition from Taipei to Peking in 1979 under the Carter administration, it made clear its plans to continue arms sales to the island. The policy was later codified by Congress in the Taiwan Relations Act, which requires the United States to provide means for Taiwan's defense.

Peking protested but went along with normalization, apparently assuming that U.S. arms supplies would diminish and eventually stop as China dropped its threats against Taiwan and began extending an olive branch to entice the island back by peaceful means.

But Peking began to have second thoughts after Reagan won the presidency. During his campaign, he proposed upgrading the unofficial U.S. relationship with Taiwan, which since 1979 has been confined to a nongovernmental office.

When Reagan announced after entering office that he would consider Taiwan's request for a new sophisticated jet fighter, Peking began criticizing the new president for violating the 1979 agreement, in which Washington recognized Taiwan as part of China.