Lawyers are deciding how much to pay families of the 290 people killed in the downing of an Iranian jetliner by a U.S. warship, but some members of Congress are not readily agreeing to make recompense.

Citing "compassion for the innocent," President Reagan authorized compensation Monday for the families of victims aboard Iran Air Flight 655, but he ruled out any reparations to the Iranian government.The United States would not admit culpability under the so-called "ex gratia," or out of grace payments that Reagan supports.

"I, from the first, have said that we are a compassionate people and I think that we all have compassion for the families of those unfortunate people that were on that plane," Reagan said during an Oval Office photo session.

A senior State Department official, briefing reporters on the basis of anonymity, said, "We'll make the payments to the families, because it's right, but we won't make any payment to Iran, because it was wrong."

The official said the Iranian authorities were culpable for sending a civilian airliner over the Persian Gulf at a time when the USS Vincennes was fighting with Iranian gunboats July 3 in the immediate area.

The State Department official said the administration eventually would have to come to Congress, either to authorize the spending of available funds or to appropriate new money.

Reagan's move to compensate the families received mixed reactions on Capitol Hill with House Speaker Jim Wright, D-Texas, predicting Congress "will be attentive to his request" if Reagan has to seek its approval of the payments.

But Rep. Tony Coelho of California, the No. 3 House Democrat, cautioned, "The payment plan is premature; there is no guarantee the aid won't flow to Iran's government, and it lets our allies off the hook."

He said the United States is providing the ships, weapons and costs of the tanker escort policy to the benefit of U.S. allies.

"One question about compensation asked by the public has not been answered by the administration: Where are our allies?" Coelho said.

Rep. Gerald Solomon, R-N.Y., charged, "Where were the calls for reparation when 240 Marines were killed in Lebanon by forces taking orders from the Ayatollah Khomeini?"