The United States and China have made "tremendous progress" toward resolving differences over implementation of a peaceful nuclear cooperation agreement, a high-level U.S. official said today.

The official said it was now conceivable that full agreement with the Chinese could be reached within weeks on guarantees from Peking that it was committed to nuclear nonproliferation.

This would permit President Reagan to submit to the U.S. Congress an already initialed nuclear accord, which has been held up for more than a year after U.S. officials and congressional critics raised questions about reports that Peking had assisted Pakistan in developing a nuclear weapons-production capability.

The official, who asked not to be identified, said that it was even possible for revised diplomatic language on nonproliferation guarantees to be agreed on by the time that China's President Li Xiannian reaches Washington on an official visit starting July 22.

The reported progress was clearly due in part to a visit here this week by Richard Kennedy, U.S. ambassador-at-large and special adviser to Secretary of State George P. Shultz on nonproliferation policy and nuclear energy affairs.

Kennedy, who arrived here Tuesday and left today, met during the course of his initially unannounced visit with Vice Premier Li Peng, a leading Chinese expert on nuclear power. A rising political figure in the Chinese government, Li Peng is to accompany President Li (no relation) during the Chinese president's forthcoming visit to Washington.

Kennedy's visit here was kept secret on Tuesday and Wednesday, but on Thursday, the Chinese offered photographers a chance to take pictures of Kennedy and Li Peng. Asked about the Kennedy visit, U.S. Embassy officials in Peking tended to play down the significance of his talks here.

An embassy spokesman said that Kennedy was responding to a longstanding invitation from the Chinese and that his visit was not related to plans for the Li Xiannian visit to Washington.

The spokesman said, however, that Kennedy was talking with the Chinese about implementing a possible nuclear cooperation agreement.

Kennedy told United Press International on Thursday that "substantial progress" had been made toward ending the deadlock over the nuclear pact.

He said that his meetings in Peking proved "productive and helpful in every respect" in clarifying Washington's stand on nonproliferation. The discussions were designed to ensure that implementation of the accord would proceed "smoothly and without misunderstanding."

Asked why Kennedy's talks here were kept secret initially, the high-level official, who asked not to be identified, said, "You can't conduct foreign policy in the newspapers. You can't deal with countries on delicate matters in that way."

This official added that "the Chinese have been very sincere" and "have made an enormous number of concessions in the direction of joining the world of nations committed to nonproliferation."

In January l984, during a visit to Washington, Chinese Premier Zhao Ziyang said China would not help other nations obtain nuclear weapons. He reiterated that assurance in a Peking speech last September.