Reagan confident on Sinai

Israelis observed the last day of Passover, the festival commemorating the ancient Jews' exodus from Egypt, amid some last-minute doubts about Israel's evacuation of the Sinai Peninsula.

The United States, concerned by a sudden chill in Egyptian-Israeli relations, intensified its efforts to iron out disputes which could delay the Sinai handover. Deputy Secretary of State Walter Stoessel was due in Israel Wednesday night to mediate between Israeli and Egyptian leaders. Defense Minister Ariel Sharon will go to Cairo today (April 15) to try to settle the frontier dispute.

President Reagan, asked by reporters if he was concerned that Israeli Prime Minister Menachem Begin might renege on a promise to return the territory, said: ''I have his pledge that the turnover is going to occur and they are going forward within the . . . framework of the Camp David talks.''

In Cairo, the US ambassador to Egypt, Alfred Atherton, said the Israeli withdrawal this month would be completed on time. Mr. Atherton made the statement to reporters after a 90-minute meeting with Egyptian President Hosni Mubarak amid speculation that Israel might delay its pullout.