President Reagan and Canadian Prime Minister Brian Mulroney have moved close to agreement on the controversial issue of acid rain, which is threatening the environment on both sides of the border, and will announce specific stands today, U.S. and Canadian officials said after the leaders' initial White House meetings yesterday.

U.S. sources said Reagan will endorse a January report on acid rain in a way acceptable to Mulroney, who has made progress on the issue his central objective in the talks here.

The report by U.S. and Canadian special envoys described acid rain as "a serious environmental problem" for both countries and called for a $5 billion U.S. "demonstration program" to find technologies to combat it.

Mulroney told Senate Foreign Relations Committee members and other lawmakers on Capitol Hill yesterday that he did not know precisely what Reagan will say about acid rain but that he was "encouraged" by his discussion with the president several hours earlier.

The administration added to robust confusion on the issue with a statement at a White House briefing immediately after the Reagan-Mulroney meeting that "we have reached agreement" on acid rain.

White House officials said later that the statement, by a senior State Department official, was premature. Other administration sources said that, at the time it was made, the agreement was limited to "a process by which we announce our positions on the [envoys'] reports."

At issue is the responsibility for generating and cleaning up sulfur and nitrogen pollutants, which rise from a variety of industrial and other causes and drift to earth on both sides of the border. Acid rain, as it is known, has become the No. 1 irritant in U.S.-Canadian relations.

Mulroney told reporters shortly before meeting with Reagan that "the fact of the matter is, your environment is dying just as surely as summer follows spring, and so is ours." He said his meetings here this week will be remembered "for the devastation inflicted on the North American continent unless action is taken, beginning today."

How much tangible action Mulroney will be able to obtain from a reluctant Reagan administration was still uncertain last night. Until now, Reagan has been unwilling to accept a conclusion that the problem is man-made, insisting privately that forest fires are to blame.

The report in January by the special envoys -- former U.S. transportation secretary Drew Lewis and former Ontario premier William Davis -- said that man-made pollutants "contribute far more to acid rain in the United States than natural sources." An extensive U.S. National Academy of Sciences study released last Friday found "a strong association" between sulfur dioxide emissions and environmental damage in eastern North America.

Reagan, welcoming Mulroney in a full-dress arrival ceremony on the South Lawn, called protection of the environment "a matter of great significance" and said the two nations "must build on what has already been accomplished" in combating air pollution.

Mulroney's remarks at the ceremony were extremely general and did not deal with any of the substantive issues being discussed.

White House correspondents were surprised when two chairs were placed on the platform from which the welcoming remarks were made, permitting Reagan to sit during Mulroney's speech and vice versa, rather than standing, as is customary.

Bruce Phillips, public affairs counselor of the Canadian Embassy, said Canada requested the chairs because Mulroney suffers from "a slight inner-ear problem" that can affect his balance if he stands for long periods.

The White House said Reagan and Mulroney have approved a five-year extension of the North American Air Defense (NORAD) agreement, which expires in May, and will sign it today. Canadian sources indicated that Mulroney will express his country's views on the 1972 U.S.-Soviet Antiballistic Missile Treaty in order to quiet fears at home that the extension could involve Canada in a missile-defense system.

Mulroney announced that Canada will accept Reagan's invitation to help develop a manned space station, at a cost to Canada of $570 million over the next 15 years.