The punitive steps announced by President Carter inaugurate a new and open-ended phase in the U.S. response to the five-month-old crisis with Iran, senior administration officials said yesterday.

In a broad sense, this is the third phase of the American effort. It has been undertaken under conditions that were described at the White House as "much more serious" than those of the past.

As described or implied by Carter's statement and the remarks of senior aides, what was announced yesterday is more important as a turning point than as a pressure point. Nobody expressed belief that the breaking of U.S.-Iranian diplomatic relations and the other measures can bring an end to the crisis, but there were broad hints that additional steps to bring additional pressures are likely.

One crucial question left unanswered is how far down the road toward military action the United States would go in this latest phase of the crisis. Carter spoke of the U.S. "patience and restraint" in the past tense, and senior aides pointedly declined to rule out military responses such as a naval blockade of Iran if the crisis continues unresolved.

However, such action might well be more difficult and dangerous now than it was last fall. A major change in the international situation at the turn of the year brought the Soviet Union into collision with the United States over Afghanistan and raised tension in the Persian Gulf, making it even less likely that Moscow would accommodate U.S. measures to bring military pressure on Iran.

There was talk in high government circles that Iran could face a threat of parallel action by the two superpowers resulting in a partition of that country, with the Soviet Union seizing the northern part near its borders and the United States and its allies taking over the oil-rich area in the south.

There was no sign that Moscow has agreed to such a constellation of interests, however. Under the present circumstances, U.S.-Soviet conflict over Iran appeared to be more likely than superpower cooperation.

In describing the background and dimensions of Carter's latest decisions, a White House official who briefed reporters divided the past history of the hostage crisis into two previous phases.

Phase one was the period from the taking of the hostages last Nov. 4 through the early part of this year. In this period the United States relied on a policy of "turning the screws" on Iran through increasingly strong non-military gestures and pressures in Washington, at the United Nations and with the European and Japanese allies.

"There was in effect no legally constituted authority in Iran that was willing to discuss in any serious fashion, even through intermediaries, a peaceful resolution of this matter . . . There was no process, no exchange back and forth, no demonstrated willingness on their part actually to engage in something other than the making of unilateral demands," the White House briefer recalled.

Phase two began with the election of Abol Hassan Bani-Sadr as president of Iran late in January, holding out the prospect of a negotiated settlement by a governmental leadership that aimed to bring the hostage crisis to an early end. "As you saw by our public posture," said the White House official, "we did agree to a process involving reciprocal actions, a United Nations commission, a step-by-step approach on our part and their part culminating in the release of the hostages. We delayed additional [punitive] steps in order to see if the process would work . . . to create the most favorable climate in which it could work."

This phase is now at an end, reporters were told, because "it did not work." Carter was forced to conclude, the official said, that Bani-Sadr and the Iranian governmental authorities could not or would not keep their commitments to full-scale action to free the hostages or even such interim steps as the takeover of control of the 50 hostages from the militants at the U.S. Embassy compound.

Ayatollah Ruhollah Khomeini's determination to reserve the decision about immediate and long-term solution of the hostage problem for the Iranian parliament, which is not likely to be in a position to act before late May or June, means that there is no functioning mechanism capable of dealing with the matter at the present time.

Phase three of U.S. policy, therefore, is a return to pressure and threats -- but in more complex international circumstances than before, and in the deepening domestic political struggles of Iran and the United States.

In Iran, the political forces of the conservative clergy have stalemated the efforts of Bani-Sadr and other secular leaders who sought to resolve the hostage crisis quickly. The hostages are more than ever pawns in Iran's domestic political dynamics, with the U.S. measures and all else beyond Iranian shores seemingly to be of only secondary importance.

In the United States, Carter's own political fortunes are increasingly at risk. There is a clear danger that the hostages will still be held as the Democratic National Convention takes place in mid-August and even as the U.S. votes in November. The grave domestic consequences of such a continuing stalemate argue powerfully for strong action in the new phase of the hostage crisis.