President Reagan has decided to offer U.S. compensation to the families of the 290 people who died aboard an Iranian airliner shot down by a U.S. warship, the White House announced Monday.

Spokesman Marlin Fitzwater said Reagan wanted to "ease the hardship of the families." But he said that no money would go to the Iranian government and stressed that the United States was acting out of humane considerations rather than any legal obligation.Less than an hour before Fitzwater appeared in the White House briefing room, Reagan tipped off reporters, saying, "We all have compassion for the families of those unfortunate people."

The spokesman stressed that the payments would be "ex gratia," or voluntary, and would be subject "to normal U.S. legal requirements, including, if necessary, action by Congress." Consequently, he said, "details concerning time, amount and other matters remain to be worked out."

Fitzwater said that the amount of compensation had not yet been decided upon.

He said the payments would be made through a third party, whom he did not name. Fitzwater did say the Red Crescent, the arm of the Red Cross in that area of the world, could serve that function.

"This offer of ex gratia compensation is consistent with international practice and is a humanitarian effort to ease the hardship of the families," Fitzwater said. "It is offered on a voluntary basis, not on the basis of any legal liability of obligation."

"We will do everything possible . . . to ensure that this compensation goes directly to the families of the victims involved," he said, "and not to the government."

"Our offer is without strings," Fitzwater said.

The spokesman said that Reagan still believes the "actions of the USS Vincennes . . . were justifiable defensive actions." But Fitzwater characterized the president as "saddened at the tragic deaths of the innocent victims of this accident."

Fitzwater said the decision was not connected with the opening of a U.N. Security Council debate on the incident and said the administration "will not countenance" any impression that it is geared toward improving relations with Tehran or winning the release of nine Americans held hostage in Lebanon by pro-Iranian elements.

Fitzwater also reiterated the U.S. position that Iran bears a "heavy burden" for

allowing the Iran Air jetliner to fly over an area where a firefight was raging between the Vincennes and Iranian gunboats.

Asked during an Oval Office picture-taking session Monday whether he had decided that compensation should be granted, the president said, "We are a compassionate people."

Asked about public opinion polls showing that a majority of Americans surveyed had said they opposed paying such compensation, the president said, "I think that they have tied that to our feelings about the government and what it did with regard to our people in the embassy there (in Tehran) when the ayatollah (Ayatollah Ruhollah Khomeini) came in."

In saying there would be no payments to Khomeini's government, Reagan said:

"We don't feel that any such thing is called for. As I've said, we all can have compassion for the innocent people who are the victims."

Asked whether sending such compensation might send a bad signal, he said, "I don't ever find compassion a bad precedent."