President Carter's special Middle East mediator said yesterday that Egyptian-Israeli talks on Palestinian autonomy have not produced "one single iota of an agreement" and added it "may be impossible" to meet the May 26, 1980, target for an accord.

Testifying before the House subcommittee on the Middle East, Robert S. Strauss conceded: "I come here as barren as a man can be on that . . . There are vast, vast differences that we don't know how to bridge. To say anything different would be a fraud."

But Strauss rejected suggestions that the United States seize on the difficulties besetting Israeli Prime Minister Menachem Begin's goverment to pressure Israel into making concessions in the talks over a self-rule system for the Palestinian inhabitants of the Israeli-occupied West Bank and Gaza Strip.

Within the past few days, the Begin government has been dealt heavy blows by the resignation of Foreign Minister Moshe Dayan and an Israeli supreme court decision inhibiting Begin's policy of establishing settlements on the West Bank.

Strauss will fly to London tonight for talks with the heads of the Egyptian and Israeli negotiating teams, Prime Minister Mustapha Khalil and Interior Minister Yosef Burg.

Some administration officials are known to have argued that Strauss should, in London, seize on these events, particularly the resignation of Dayan, and advocate of a more flexible Israeli approach, to press for quicker progress in the autonomy talks.

However, in his testimony yesterday, Strauss said: "I think the worst mistake we could make right now is to be so shortsighted as to push too hard when an ally is having internal problems . . . If we inject ourselves, it's going to be resented."

His comment appeared to align Strauss with those in the administration who believe that attempting to pressure Israel at this point would be counterproductive. But, some informed sources said there is a consensus within the administration that Dayan's resignation could trigger a reassessment of Israeli policies and that the United States should discreetly try to influence the process.

Strauss referred to that indirectly when he said, "Dayan's resignation is obviously going to cause a rethinking of things in Israel, but we don't know where it's going to come out."

As one source put it, "Strauss will be watching for openings and nudging and using his powers of persuasion in London. But he's not going to make headlines by publicly wagging his finger at Burg and shouting that Israel should listen to Dayan and be more flexible."

In response to comments by the subcommittee chairman, Rep. Lee Hamilton (D-Ind.), who said he was not satisfied with the pace of the negotiations, Strauss said neither Begin nor Egyptian President Anwar Sadat has wanted talks to move faster until now.

He said the two sides are far apart on the central issue -- the degree of self-rule granted the Palestinians -- and added he would consider it a success if 50 percent of the issues are resolved by may.

"In spite of our best efforts," Strauss warned, "it may be impossible to achieve a neatly packaged final agreement by May, although that is still our target."

Still, he added, the United States sees some "encouraging areas of potential agreement." And he said he plans in London to engage in extensive "pulse-taking" with Khalil and Burg to seek agreement on a "decison-making process" that will begin resolving some issues.