Israeli government assured the U.S. embassy today that bulldozing work going on at eight places in the northern Sinai is not for new settlements but to prepare farmland for existing ones.

The U.S. embassy had sought clarification of reports that the ground-breaking was for eight new settlements.

The issue is extremely sensitive surfacing before the resumption of Israeli-Egyptian talks on the future of the Sinai, among other subjects. Israel captured the Sinai Peninsula from Egypt in the 1967 Arab-Israeli war.

The Israeli government is under tremendous pressure to show that it will not abandon the settlements, most of which were established under official auspices with public money.

In a gesture apparently designed to ease some of the settlers' anxiety, Prime Minister Menahem Begin and his wife applied for membership in a settlement called Neot Sinai overlooking the Mediterranean Sea.

The application was made last month, about two weeks after Egyptian President Anwar Sadat came to Israel, and when the settlers were beginning to express their worries. Confirming the report, the settlement manager Yitzhak Regev, said, "We know Begin well. If he comes to the point that he has to offer Egyptian severeighnty over our (settlement) then we are sure that he will also worry about providing Israeli security and services."

There are unconfirmed reports that the Israelis are prepared to offer Egypt a swap involving a chunk of desert in the southern Negev Desert in exchange for the Rafiah region in the northern Sinai, where most of the Israeli settlements are concentrated south of Gaza.

Foreign Ministry officials said they had never heard Foreign Minister Moshe Dayan mention the idea. But former Foreign Minister Abba Eban advocated - in a newspaper article today.

Eban went on to ridicule a statement by the head of the settlement department of the Jewish Agency that the government should go ahead with previous plans to build 2,000 new residential units in the Rafiah area. There does not, in any case, seem to be great enthusiasm for moving to an area that may soon be Egyptian-ruled. An offer this week of 125 building lots in one settlement was met with five applications.

An Israeli government announcement that was apparently related to the clarification given to the United States noted that the Israeli proposal to Egypt stipulates that the Israeli settlements in the Sinai "will remain where they are" and that "the preparation of land for farming" is "in accordance with" the peace proposal.

"There is reason whatsoever that improvement plans should not continue." A Foreign Ministry spokesman said.

Although Begin has insisted that the future of the Israeli settlements is assured, the settlers continue to express doubts. A week-long vigil of settlers and their allies is to start Sunday outside the Prime Minister's house.

This skepticism persists despite Begin's pledge to the Parliment Dec. 28: "Jewish settlements will remain in place. These settlements will be linked with Israel's administration and courts. They will be protected by an Israeli force."

Egypt has rejected this however. And Abba Eban said today. "Begin cannot have peace both with Egypt and with Gush Emunim." Gash Emunim is the most vocal of the groups dedicated to continued Israeli settlement in the occupied Arab territories.

Begin's own party, Herut, applied today for permission to establish three new settlements in the Rafiah salien. Approval of such applications is ordinarily a lengthy process.

Education Minister Zevulun Hammer, a Herut member who is known to have placed public curbs on his private doubts about the Begin peace offers, said that he is convinced that no existing settlement will be abandoned. He said that the government is "walking a tightrope" and that its supporters must "watch that peace-making does not contradict settlement."

The ambivalence of the government about the settlement was perhaps best summed up by Oayan upon his his return from Egypt with Begin. The foreign minister said the he felt every threat to the future of a settlements like a stab in the back. A week later he told worried setters in Yamit. "If you stand in the way of peace, the country is not going to back you."