The federal government agreed yesterday to back scaled-down Indian claims for several million acres of land in Maine.

The Justice Department announced in a federal court in Portland it would support litigation for a substnatial share of the two Indian tribes' claims, although the amount of land involved has been reduced through negotiation.

Although exact figure s could not be determined yesterday, the claims could total at least 5 million acres.

Maine's congressional delegation, however insisted it would proceed with legislation to extinguish the tribal claims, although leaving a door open for the Indians to be compenated through civil suits.

Meanwhile, the White House moved into the growing controversy of tribal land claims for the first time by saying claims for the first time by saying President Carter would soon appoint a special representative to mediate the dispute within the next few months.

Several observers sadi the ultimate resolution must come from Congress in the form of a sizeble cash settlement for the Indians with federal funds. In the end, they predicted, very little, if any, land would be transferred to Indian ownership and no small landowners would be required to surrender title to their properties.

That cash solution, it was said, probably would become a model for similar existing and potential Indian claims in toher states ranging from Vermont to South Carolina.

THe two Maine Indian tribes - Penobscots and Passamoquoddies - originally had claimed rights to 12.5 million acres theysay were illegally taken from them in violation of a 1790 act of Congress. A federal court has ordered the U.S. Department to fullfill its trust relationship to the two tribes.

Intially, the Interior Department had agreed to defend 10.5 million acres of the Indian's claims, but that amount, the Justice Department said yesterday, has been reduced through negotiations with the Indians.

The governemnt now will support claims only for land actually uesd and occupied by the tribes in 1790. That would exclude from challenge most of Maine's populous and financially valuable coastal territory previously claimed by the Indians.

Tribal officers yesterday estimated that the new claims amounted to between 5 million and 8 million acres. The Justices Department declined to estimate how much was involved but did not disagree with the Indians' assessment.

THe issue has been an awkward one in Maine, where the Indians were at first demanding about 60 per cent of the state's land area. An estimated 350,000 state residents would have been affected under that large a claim. Under the scaled-down version, between 75,000 and 90,000 residents would be affected.

The state government had refused to negotiate with the tribal representatives and instead has sought a solution through the federal government. The claims have cast a legal shadow over thousands of land titles in the state, and at one time a state agency was unable to sell municipal bonds for those towns and cities where claims were pending.

Gov. James Longley and the Maine congressional delegation met yesterday to discuss the claims with Peter Taft, an assistant attorney general who has handled the negotiations.

Longley called the latest Justice Department plan "a step forward" because it reduces the amount of land claimed. But he and Maine's congressional leaders insisted they would press legislation today that would extinguish the Indians' legal title to the properties. That, they hope, would remove clouds from the land titles and reassure residents who fear being uprooted.

According to Rep. David Emergy (R-maine), the delegation's bill will state that the Indians would be entitled to press for trespass damages for the land they claimed.

Sen. Edmund S. Muskie (D-Maine) said that the federal governemnt must bear ultimate financial responsibility for settling the issue. He said the United States had failed to meet its trust responsibility established by the 1790 act, which declared that no Indian territory could be taken by anyone without specific consent of Congress.

It must be settled, Muskie said, without compromising the rights of Maine's homeowners.

"I know of no way you can force 350,000 people off of their property," Muskie said. "They bought that land on the basis of title searchesthat would be valid in any state in the union."