The Reagan administration yesterday reaffirmed the longstanding U.S. position on dealings with the Palestine Liberation Organization, even while under increasing pressure to support Palestinian rights and aspirations more actively.

The reaffirmation of the U.S. refusal to deal with the PLO under existing circumstances came from State Department spokesman Dean Fischer in response to questions about a letter last week from President Reagan to Israeli Prime Minister Menachem Begin.

As reported by John Scali of ABC News and confirmed by independent sources, Reagan forcefully objected in the letter to continuation of Israeli checkpoints in Beirut that impeded the travel of Lebanese Prime Minister Shafiq Wazzan, who is acting as an intermediary between U.S. special envoy Philip C. Habib and the PLO.

If the prime minister could not carry on with his intermediary role, the United States would have to find "other means" of obtaining PLO views, Reagan's letter, presented to Begin last Wednesday, reportedly said. This appeared to suggest that the United States might be forced to deal directly with the PLO.

The Israeli leader answered the following day. Without reference to potential U.S. contact with the PLO, Begin reportedly told Reagan that the passage had been opened so that Wazzan could travel unimpeded in his diplomatic endeavors around Beirut.

In September, 1975, then-Secretary of State Henry A. Kissinger pledged to Israel that the United States would not recognize or negotiate with the PLO until that organization recognized Israel's right to exist and accepted U.N. resolutions 232 and 338.

The pledge, which all subsequent administrations have honored and Fischer reaffirmed yesterday, has not been construed to cover indirect negotiations, such as those Habib conducted in arranging the Palestinian-Israeli cease-fire a year ago, and those he is conducting through the Lebanese government and other diplomatic parties in Beirut.

On Capitol Hill, Acting Secretary of State Walter J. Stoessel is reported to have been the target of pleas late yesterday from several members of Congress for a stronger and more forthright U.S. position on the Palestinian question, especially on the West Bank and Gaza.

The lawmakers, who attended a closed-door briefing by Stoessel on Lebanon, argued that a strong U.S. position aimed at Palestinian rights and a Palestinian entity on the West Bank could help bring long-term gains from the bloody conflict and might even help resolve the conflict.

The PLO is reported to be asking for substantial political concessions to make palatable a decision to withdraw its leadership and troops from Beirut.

France and Egypt, which have been in close contact with PLO officials, have drafted a U.N. resolution endorsing Palestinian rights, including self-determination. There are indications that this is intended to play a role in the political maneuvering about a Beirut settlement.

Throughout the course of the current Beirut negotiations, Saudi Arabia is believed to have been calling for a shift of U.S. policy toward the Palestinians.

Israel, on the other hand, has strongly resisted any suggestion of dealings with, or approval of, the PLO. Begin, in his letter to Reagan, is said to have rejected anew a proposal that the PLO be allowed a political office in Beirut after its military elements depart. Begin had previously been informed that Washington did not object to such a proposal as long as the Lebanese government approved.

In a related matter, the administration took a more cautious approach yesterday to the assigned mission of 800 to 1,000 U.S. Marines who may go to Lebanon as part of a multilateral force.

The initial public statement on the force from the White House a week ago set out two missions: to assist Lebanese armed forces in the "orderly and safe departure" from Beirut of armed PLO fighters "and to assist in the transition to authority of the Lebanese government in Beirut."

The official statements yesterday, like those on television Sunday by Secretary of Defense Caspar W. Weinberger, limited the Marines' mission to overseeing the PLO's departure.

The State Department statement said the presence of the international peacekeeping force "would facilitate" establishing Lebanese government authority, but this was no longer described as one of the missions of the Marines.

In a matter likely to run into heavy congressional fire, the State Department said the administration planned to report to Congress on the Marines under a section of the War Powers Act that does not set a time limit on their presence.

The chairmen of the Senate and House committees on foreign relations, Sen. Charles H. Percy (R-Ill.) and Rep. Clement J. Zablocki (D-Wis.), have said recently that such a report will be unacceptable.