Indian Prime Minister Rajiv Gandhi, ending three days of talks with the Reagan administration, yesterday turned aside a concerted U.S. effort to win his backing for the "Star Wars" antimissile defense plan.

Speaking to the National Press Club as his Washington visit neared an end, Gandhi said he was "still not totally convinced" by a special U.S. briefing that the Strategic Defense Initiative is suitable to the world today.

The Indian leader, following discussions with Secretary of Defense Caspar W. Weinberger, SDI director Lt. Gen. James A. Abrahamson and special arms adviser Paul H. Nitze, said that "Star Wars" could mean "handing over to a machine" split-second decisions about nuclear war because of the unprecedented complexity involved.

Gandhi went on to refer to the SDI plan as "a totally new dimension to the arms race" which will make disarmament more complicated and difficult.

Earlier, Gandhi told reporters the removal of human beings from a computer chain of SDI actions is "very dangerous."

Despite their continuing differences on "Star Wars," U.S. arms for Pakistan and a number of other issues, both Gandhi and his U.S. hosts expressed satisfaction about the new Indian leader's visit, which ends in a day-long trip to Houston today.

"We look at the world from different angles . . . but after my conversations with U.S. leaders we feel we can cooperate to reduce the differences," Gandhi said at the Press Club.

A State Department official said Gandhi came across as "a very thoughtful and excellent listener" and "very much his own man" among more experienced advisers. The official said Gandhi's evident popularity with the administration, Congress and the public will be helpful in improving often rocky U.S.-India relations.

Gandhi, while insisting publicly that he has not changed India's existing position on Afghanistan "at all," gave the impression to U.S. officials that he is still considering undertaking a more active diplomatic role on the issue.

The Indian leader told the Press Club that he believes U.S. leaders "are keen on a solution" to the five-year Afghan war. He added, evidently on the basis of his five-day visit to Moscow last month, that "We believe the Soviets are also keen to come to some sort of conclusion on this."

"If both the countries can get together and sort it out it would do all of us in our region and in the world a very great favor," he added in a remark which seemed to shy away from a role for India.

A State Department team headed by Assistant Secretary of State Richard W. Murphy is to meet here next Tuesday and Wednesday with a Soviet team headed by Yuri Alexeev, director of the Soviet Foreign Ministry's Middle East department, for the first full-scale discussion of Afghanistan between the two countries in nearly three years. There is no expectation of a breakthrough in these talks, U.S. officials said.

United Nations-sponsored "indirect" talks involving the Afghan and Pakistan governments are to resume in Geneva later next week.

Following a meeting with Weinberger, Gandhi said no "arms deal" was discussed during his visit to Washington. He described a U.S.-Indian arms supply relationship as "something we can build up over a period of time, slowly."

U.S. sources said Gandhi was told by Commerce Secretary Malcolm Baldridge Thursday that an administrative process has been established to streamline approval of U.S. exports of civilian high technology to India. This follows a high technology memorandum signed by the two nations last month. Officials compared the expedited procedure for high-tech sales to India with similar procedures which have been established for U.S. sales to China.