The United States expect Egypt to participate in a new round of negotiations with Israel two weeks from now despite an apparent hardening of President Anwar Sadat's position.

U.S. diplomatic officials said yesterday that Secretary of State Cyrus Vance is still planning to arrive in the Middle East on Aug. 5 and then meet separately with Israeli and Egyptian officials in Jerusalem and Cairo.

Those talks are to be followed by a resumption of the foreign ministers meeting begun in Leeds Castle, England, 12 days ago. It is expected that the latter discussion will take place about Aug. 9 at a U.S.-administered observation post in the Sinai.

U.S. Special envoy Alfred Atherton left here for Cairo yesterday after talks with Israeli Prime Minister Menachem Begin and Foreign Minister Moshe Dayan. He plans to spend three days in Cairo before returning to Jerusalem.

Atherton, according to senior U.S. officials, is confident he will win a firm commitment from Sadat that Egyptian Foreign Minister Mohammed Kamel will attend the Sinai meetings.

On his arrivial in Cairo, Atherton told reporters, "The United States is looking for ways to continue the negotiating process. We are continuing despite the many difficulties that remain."

Atherton described his meeting with Begin yesterday, "very detailed and useful." He added, "I think I'll be in a position to convey a better understanding of Israel's position to the Egyptian leaders."

Before Atherton left Jerusalem, senior U.S. officials said the State Department is still considering whether Vance should make the trip to Cairo and Jerusalem without an Egyptian commitment to participate in the Sinai talks.

U.S. officials, however, were confident that Egypt will come around despite Sadat's declaration Thursday night in a sharp attack on Begin's policy that Egypt would not negotiate with Israel unless new proposals are presented.

In his two-hour address in Alexandria, Sadat said he would fight Israel "until the end of the world" unless Israel return Egypt's "stolen lands," meaning those territories occupied in the 1967 war.

The previous day Sadat had ordered Israel's nine-member military negotiating team to leave the airbase at Alexandria where they have been serving as a go-between the Egyptian and Israeli foreign ministries.

Sadat took the tougher line after Prime Minister Begin rejected his suggestion that norther Sinai town of El Arish be returned to Egypt as a goodwill gesture. At the time Begin said, "Nobody gets anything for nothing."

Begin was quoted in a newspaper interview as saying he was not too concerned about personal attacks made against him by Sadat and other Egyptian leaders.

"They are to be regretted," he said. "They are cheap and vulgar but must not stand in the way of continuing our effort to reach a mutually satisfactory agreement. That is what is really important."

U.S. officials said that while there has been a rise in the level of rhetoric, they detected no significant back-tracking on Sadat's part, adding that they felt the Egyptian president is still determined to keep the negotiations going.

Israeli Foreign Ministery officials also said they were hopeful the peace talks will be held. They expressed concern, however, that Sadat may be engaged in a "war of nerves" that is designed to hold the peace talks in doubt until Vance arrives in the Middle East.

Referring to the expulsion of the Israeli military delegation, a Foreign Ministry spokesman said, "Even if that was not a major event by itself, it may be part of a whole series of acts with a purpose in mind."