Retiring U.S. Ambassador to Syria Talcott Seelye, one of the State Department's most experienced Middle East experts, said today that the time has come for the United States to establish a relationship with the Palestine Liberation Organization if it is serious about seeking a comprehensive peace in the Middle East.

In an interview with several correspondents only hours before his departure from his post in Syria, Seelye urged both "a dialogue" with the PLO and a new peace-seeking initiative from the United States to replace the moribund Camp David peace process set up by Washington, Israel and Egypt. The Camp David effort, he said, had become a self-defeating "red flag" for other Arab governments in the region who would have to participate in any peace agreement.

Seelye is retiring after more than three decades in the foreign service because the Reagan administration did not offer him a new post commensurate with his rank and experience.

"To really find a solution to peace in the region we must start having relations with the PLO -- a dialogue," the lanky State Department veteran of 32 1/2 years service said as he reposed on an easy chair in his embassy office in Damascus for the last time. "It would strengthen moderates in the PLO and demonstrate to our Arab allies, such as Saudi Arabia, that we are serious about dealing with the issue of the Palestinians."

In making the statements on the record, Seelye joined a host of other former U.S. officials who have recently expressed the need for the United States to begin dealing with the PLO. Only recently, former president Carter's national security adviser Zbigniew Brzezinski openly advocated the same thing, even though he never had when he was in office. Seelye, one of the few fluent Arab speakers among senior foreign service officers, dismissed arguments raised recently by Secretary of State Alexander Haig Jr. that contacts with the PLO were taboo unless the PLO officially recognized the state of Israel. Haig cited a commitment made in 1975 to Israel by then secretary of state Henry Kissinger.

"The 1975 Kissinger commitment did not preclude a dialogue with the PLO, it only precludes negotiations and recognition," Seelye said, puffing on a thin cigar as he reviewed his long service in the Arab world. "That commitment is not a treaty and not a legal document."

Seelye said he doubted there could be any serious progress toward a Middle East settlement as long as Prime Minister Menachem Begin remained in power in Israel, because, "In my view it is impossible to expect Begin to divest Israel of the occupied West Bank of the Jordan River and secondly because he is totally blind to the Palestinian imperative in any peace agreement."

Seelye, who was born in Beirut where his father was the president of the American University of Beirut, has served in Jordan, Kuwait, Saudi Arabia, Lebanon, Tunisia and Syria, as well as two tours in the Near East office of the State Department. Sources here said the Reagan administration's failure to offer Seelye an important post followed pressures from the Israel lobby in Washington, which reportedly considered him too sympathetic to the Arabs.

Asked how the United States might initiate a dialogue with the PLO, Seelye said simply: "You just have to pick up the phone."

Seelye indicated that he was worried about whether the time had already passed when a real compromise between the Arabs and Israel was possible. He said Begin's policy of establishing armed Jewish settlements in occupied Arab territory on the West Bank had reached a point where "these plantation settlements are so widespread that it might be too late. If Israel was forced to withdraw from them it could cause a political upheaval in Israel itself."

He argued for a U.S. policy that would include a strong position against Israeli settlements on Arab territories and make clear to the Israelis that the United States would not tolerate continued Israeli entrenchment in occupied Arab lands.