The Reagan administration is now focusing its diplomatic efforts in the Lebanon crisis on persuading Syria to take in the Palestine Liberation Organization (PLO) fighters now under siege in Beirut.
A White House official said that ''the major impasse'' in the Lebanon negotiations at the moment was the unresolved question of where the PLO would go.
Officials both at the White House and in the State Department, meanwhile, tended to play down as virtually insignificant a statement made in Paris recently by Isam A. Sartawi, an adviser to PLO chairman Yasser Arafat. Sartawi said that the PLO was prepared to recognize Israel ''on a reciprocal basis.''
''The bottom line is that we have to have a more authoritative statement,'' said a State Department official, who indicated that a clear declaration on the subject from Yasser Arafat himself was required before the US would consider dealing directly with the PLO.
An Israeli Embassy spokesman, Nachman Shai, said that the PLO's tactic in the past has been to issue a ''so-called moderate statement'' only to issue a ''counterstatement'' the day after. He said that recent statements by Arafat and Sartawi had been ''vague and ambivalent.''
Uri Avnery, publisher and editor of the Israeli weekly Haolam Hazeh, met recently in Beirut with Arafat and quoted him as saying that the PLO would accept, if not a ''democratic secular state,'' then two separate states including Israel.
Mr. Shai said, however, that a full reading of the Arafat-Avnery conversation showed ''nothing new'' and does not specify the kind of Palestinian state the PLO would accept.
What is clear is that US officials want to keep the focus on persuading Syria to take the PLO. They thought that the Syrians had agreed to this only to back off once they considered the dangers involved in having as many as 6,000 PLO fighters stationed on their soil. Officials now think that Saudi Arabia could play a key role in getting Syria to reconsider.
The unknown factor is the newly appointed secretary of state, George P. Shultz, who has yet to make his views on the PLO signals known. A congressional expert said, ''I think the messages coming from the PLO are significant. But nobody at the White House or State Department is going to say so yet. I think they are waiting for Shultz.''