President Carter yesterday underscored his recent conciliatory stance toward Iran by promising that if the American hostages are freed, he will release the Iranian assets frozen in this country, lift restrictions against shipments to Iran and seek normal relations between the two countries.

Carter's pledge was his most explicit statement to date of what he is prepared to do if Iran cooperates in resolving the hostage problem. It came as Secretary of State Edmund S. Muskie, in a companion move, made what administration sources called "a very carefully and precisely drawn statement" of how the United States believes the war between Iran and Iraq should be resolved.

Speaking in Chicago, Muskie said the war "can and must be resolved" on the basis of two principles: "that territory must not be seized by force of arms" and that "neither side should seek to interfere in the affairs of the other."

Administration sources, elaborating privately on Muskie's remarks, said they were intended to signal three main points: that Iraq should withdraw its invading forces from Iran, that Iran should cease trying to incite Iraq's Shiite Moslem minority against the country's Sunni Moslem leadership and that both countries should submit their dispute over contested border territory and waterways to mediation or arbitration.

The sources said the United States is exploring this approach privately with individual governments and international organizations that might have influence with the combatants. They added that Muskie's speech was intended to lay out for the first time the elements that Washington believes must be the basis for any resolution of the war, while preserving the U.S. stance of neutrality in the conflict.

In that respect, the sources denied that there is any contradiction between Muskie's proposal and statements by Carter and other senior administration officials that have been interpreted as tilting toward Iran in an effort to get the hostages freed.

In reality, the sources said, the United States has an equally pressing interest in ending the plight of the hostages and in halting a war whose prolongation could seriously unsettle the Persian Gulf region with its vital oil supplies. For that reason, the sources continued, the two out-front elements of U.S. policy -- resolving American differences with Iran while trying to find a formula for ending the war -- should be viewed not as contradictory aims but as coordinated efforts to restore the stability of the Gulf area.

The sources noted that the comments made by Carter yesterday during a campaign stop in Youngstown, Ohio, actually did little more than spell out explicitly things at which he has hinted several times in recent days and that already had been stated publicly by Muskie. According to the sources, the main significance of yesterday's remarks was that they came from the president himself.

In response to a question about future U.S.-Iran relations if the hostages are freed, Carter said: "If Iran should release the hostages, then I will unfreeze the assets in banks here and in Europe, drop the embargo against trade and work toward resumption of normal commerce with Iran in the future."

"It is to our advantage to see a strong Iran, a united Iran," the president added.

Muskie, in his Chicago speech, bore down even harder on that point. He said: "We are opposed to the dismemberment of Iran. We believe that the cohesion and stability of Iran is in the interest of the stability of the region as a whole."

It is frequent rhetoric like that from senior administration officials that has triggered talk of a U.S. tilt toward Iran. If Iran could regain control over the approximately $8 billion in assets frozen by Carter after the outbreak of the hostage crisis and buy the spare parts it needs for its largely American-made military equipment, it would be in a much better position to pursue the war against Iraq.

But, administration sources cautioned, talk of resumed U.S. trade with Iran, even trade potentially involving spare parts, has to be viewed in conjuction with the parallel effort to end the fighting, and they added, that was the main message Muskie was attempting to get across yesterday.

While asserting that "the integrity of Iran is today threatened by the Iraqi invasion," he also added: "We note that the government of Iraq has consistently stated that it has no claims to Iranian territory."

"Let us also reaffirm another principle that will be essential to a peaceful resolution of this conflict," Muskie continued. "It is the principle that neither side should seek to intefere in the affairs of the other."

He concluded: "If Iran and Iraq are prepared to respect both of these principles -- that territory must not be acquired by force, and that no state should seek to intervene in the internal affairs of another -- it should be possible to bring this needlessly prolonged conflict to an early end. It should be possible to do so in a way which does justice to the legitimate concerns of both sides."