Confronted by demands from their own political leaders to be tougher in bargaining than they have been so far, Egyptian and Israeli negotiators interrupted their talks on a peace treaty yesterday while the Israelis sought new guidelines from Prime Minister Menachem Begin in Canada.

Carrying word that Egypt has responded to Israeli objections to a draft treaty tentatively agreed to last week by also escalating its demands, Foreign Minister Moshe Dayan and Defense Minister Ezer Weizman flew yesterday to Toronto, where Begin is visiting.

The new interruption in the talks - described by Secretary of State Cyrus R. Vance a week ago as all but completed - added to an already mounting sense of frustration at the White House, where doubts about the timing of the signing of a treaty are beginning to turn into doubts about whether a treaty can be signed at all, White House sources said.

Earlier White House hopes that the treaty would be signed by Nov. 19 to mark the first anniversary of Egyptian President Anwar Sadat's trip to Jerusalem now have been abandoned, and some of President Carter's aides think the fall back date of the first week in December also is rapidly slipping away.

Israeli refusal earlier in the week to accept any reference in the draft treaty's preamble to subsequent Arab-Israeli negotiations on the future of the West Bank territory apparently has led to the tougher Egyptian demands and to the new questions being raised at the White House about Israeli's willingness to live up to what President Carter thought was agreed to at the Camp David summit in September.

"I think for the first time here there has begun to be a gnawing concern about the outcome of the negotiations," a White House official said, adding, "We still believe we're going to work out the differences."

Carter hinted at these doubts Thursday during a press conference in Kansas City, Mo., where he was implicitly critical of the Egyptian and Israeli leadership for not going along with the texts worked out in Washington during a month of negotiations.

"When they refer the text back to the leaders at home in Egypt and israel, sometimes the work that has been done in partially undone," Carter said, adding, "I think that the present language as approved by the negotiators is adequate."

That point was underscored later Thursday when Egyptian negotiators just back from meetingwith Sadat at home told Dayan and Weizman that Sadat was now demanding not only the vague language that originally had been included in the draft preamble, but also commitments to a specific timetable for negotiations on the West Bank and the implementing of a local autonomy plan for the Palestinian-inhabited territory, according to diplomatic sources.

Although Dayan and Weizman were due to return to Washington last night, it was not clear when they again would meet with the Egyptians.

The next effort to break through the continuing disagreements may occur tomorrow evening when Vance flies to New York to meet with Begin during a brief stopover by the Israeli leader on his way home.

Some diplomatic analysts here saw the new snags as predictable hardening of positions as both sides press for maximum advantages and for constituency-pleasing postures in the final stages of controversial bargaining. In addition to the diplomatic "linkage" questions, arguments over billions of dollars in aid and oil revenue continue to be unsettled, although Israel wants them resolved before a treaty is signed.

But U.S. officials were clearly worried about the tone of remarks Sadat made yesterday and by Begin's failure earlier in the week to get his cabinet to go along even with the weak commitment in the draft preamble to West Bank talks.

"I will not be astonished" if the talks are suspended, Sadat told reporters at al Kantara, an Egyptian town near the Suez Canal. "Believe me, after the latest Israeli attitude, one can never say anything." Sadat's decision to increase his demands came after strong criticism of Egypt at the Arab summit in Baghdad.

Begin, who is ending an official six-day visit to Canada, said that the negotiations could continue for weeks and that Israel was prepared to continue talks past the Dec. 17 deadline he and Sadat set at Camp David for a treaty.

"We have problems," Begin said at a news conference in Toronto, but he declined to label the new impasse as a crisis.

Dayan and Weizman were believed to have briefed Begin on the Egyptian proposals brought back to Washington by acting Foreign Minister Boutros Ghali Thursday. Before leaving for Canada, Dayan described those proposals as "a new list of requests and demands" over the West Bank negotiations.

"Our position is that there shouldn't be a linkage or even a mixture of the discussions." Dayan said. ". . . we should do one thing at a time."

State Department spokesman George Sherman described the Egyptian proposals as "new ideas for handling old issues." Conference sources said the Egyptians were seeking more specific commitments on linkage in a letter that the two countries are to exchange after the treaty is signed.