President Carter's two-day peace mission here left important issues unresolved but made some progress toward a peace treaty through what appears to be a new U.S. approach to the Middle East negotiations.

Authoritative Egyptian sources said some issues were left unresolved because there was not enough time to discuss them, but others went unresolved because no agreement was reached. Still among the troublesome points, Egyptian and American sources said, were Israeli access to Egyptian oil, the timing of full diplomatic relations and the relationship-between Egypt's treaty obligations to Israel and its mutual defense commitments to other Arab nations.

The overriding issue, however, was the same as it has been since the Camp David agreements lust fall -- the future of the Palestinians in Israelioccupied territories. On that crucial question, Carter signaled a new approach that amounts to a fundamental change in the way the negotiations are being carried out -- a change reportedly acceptable to Sadat but unpopular with his top advisers.

There were strong indications that Carter was trying to induce Egypt to soften its demand that full peace between Egypt and Israel be made contingent on implementation of an autonomy program in the Israeli-occupied territories. He appeared to be telling Sadat that if the Egyptian leader signs a treaty that gives him less than he wants on this point, the United States promises to ensure that the Palestinian autonomy envisioned in the Camp David agreements is carried out.

Though Carter stressed today that all involved -- he, Sadat, and Israeli Prime Minister Menachem Begin -- are aiming at a comprehensive, regional peace between Israel and all its Arab neighbors, he also indicated that this would have to be achieved in stages, beginning with an Egyptian-Israeli treaty.

He even went so far, in a statement to the press, as to say that the overall objective was "a full and comprehensive peace involving the realization of the rights of those who suffered so long and a step-by-step progression toward peace between Israel and its neighbors."

"Those who suffered so long" was taken as an allusion to the Palestinians.

The phrase "step by step," associated with the Middle East diplomacy of former secretary of state Henry Kissinger, had been banished from the lexicon of the peace negotiations because it signified the opposite of a "comprehensive" regional settlement. Its reappearance in a statement by Carter was a dramatic signal that the American approach to the talks has changed once again, and that the Camp David objective of bringing together gether Israel, Egypt, the Palestinians and Jordan in one set of interlocking agreements has been dropped as unattainable.

Carter also indicated as much in his address to the Egyptians parliament.

"Our current efforts to complete the treaty negotiations," he said, "represent not the end of a process but the beginning of one, for a treaty between Egypt and Israel is an indispensable part of a comprehensive peace. I pledge to you today that I also remain personally committed to move on to negotiations concerning the West Bank and Gaza Strp and other issues of concern to the Palestinians and to future negotiations with the other neighbors of Israel."

The problem with this approach is that Sadat has insisted from the day he began his peace initiative by traveling to Jerusalem in 1977 that he would never accept anything that could be construed as a separate peace.

He was accused by the other Arabs of doing just that when he signed the Camp David agreements. But those at least provided for transition from Israeli military rule over the Palestinians to some form of self-government and for Jordanian participation in negotiations.

If Sadat now signs a treaty containing anything less than a specific timetable for local autonomy -- a timetable hw wants and Israel refuses to give -- he will face more denuciations and perhaps sanctions from the other Arabs.

Under the Camp David agreements, a five-year transition period leading to local self-rule begins only when elections are held among Palestinians of the West Bank and Gaza. If Sadat does not pin down the date of the elections, he is vulnerable to the charge of selling out the Palestinians by allowing the Israelis to procrastinate idefinitely.

According to informed Egyptian sources, Carter induced a reluctant Sadat to sign the Camp David accords in the first place by making a personal commitment to see that sections dealing with the Palestians were impelmented. Today he said, "I feel a personal obligation in the regard."

Whether that personal commitment would be enough to convince Egyptian skeptics -- notably Prime Minister Mustafa Khalil, a key negotiator -- is unclear. One top-level Egyptian said yesterday that what is being proposed in "obviously a bilateral peace. It's useless to pretend it's comprehensive."