As Congress began its first full inquiry into the Iran hostage agreement negotiated by the Carter administration, Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.) disclosed yesterday that President Reagan has decided to "fully implement" the terms of the deal that freed the 52 Americans.

Percy made the announcement as his committed began two days of hearings on the hostage-release agreement with testimony from two Carter administration officials and urged that it be implemented.

Both former secretary of state Edmund S. Muskie and former deputy secretary of state Warren Christopher said the terms of the agreement served U.S. interests while denying any gains to Iran from the 14 1/2-month hostage crisis.

"Iran achieved none of its major objectives," Muskie said.

There was no official confirmation of Percy's announcement by the White House or the State Department. But the Illinois Republican, who praised Reagan for deciding to honor the terms of the agreement, said he and other congressional leaders had been informed of the president's decision by Secretary of State Alexander M. Haig Jr. during a foreign

Since taking office on the day the 52 hostages were released, Reagan has refused to say whether he would honor all the terms of the agreement while his administration studied the legal aspects of the hostage deal. A special government task force that reviewed the agreement reported to the White House on Monday that it could find no legal obstacles to implement its terms.

The agreement that the administrationhas agreed to implement calls for the freezing of any assets found in the United States of the late shah Mohammad Reza Phlavi and his family, the fighting of several hundered lawsuits by American claimants seeking payments from Iran, and the transfer of about $4 billion in frozen Iranian assets, some of which would go to Iran and some of which is to go into an escrow account for the repayment of American claims as decided by a special claims commission.

There will almost certainly be a Supreme Court test of the agreement before all its terms are implemented. A Federal judge in Texas set the stage for such a test last week when he ruled that President Carter had exceeded his constitutional powers in ordering the return to Iran of frozen Iranian assets against which there are also private American claims

Muskie and Christopher told the Foreign Relations Committee yesterday that the Carter administration's Justice Department had concluded that the president had all the authority he needed to order the transfer of the assets and said they were pleased that the Reagan administration had reached the same conclusion.

There was general agreement among committee members that the administration should honor the terms of the agreement. The only strong note of criticism came from Sen. S.I. Hayakawa (R-Calif.), who complained that the agreement did not condemn Iran's taking of the hotages and made the United States sound like "the villian" in the crisis.

"We were trying to work out an agreement, not an indictment," Muskie replied.

In defending the agreement, Muskie argued that Iran "paid dearly" for taking the hostages. "Internationally and domestically, the United States emerged stronger and Iran emerged weaker" from the crisis, he said.

Christopher, the chief U.S. negotiator during the crisis, reviewed the details of the agreement and stressed the number of concessions that Iran had sought but which the United Statesrefused to make.

This country, he said, did not aplogize to Iran, paid it no ransom, did not take sides in the Iran-Iraq war and did not return either the shah or his wealth to Iran.

In addition to defending the substance of the agreement, Christopher argued that a refusal by the Reagan administration to implement it would be "a breach of faith" with Algeria, Britian, West Germany and other countries that acted as intermediaries during the negotiations. U.S. allies "would be stunned if we didn't go forward with this," he said.

Christopher also said there is ample reason to believe that Iran will live up to its end of the agrement by maintaining a minimum balance of $500 million in an account to be used to pay claims against Iran found to be valid by the international claims commission.

"Unless Iran decides to opt out of the world financial community, it will be in their best interest to fulfill the terms of the agreement," he said.