Just 48 hours after announcing his resignation as the commander of the Israeli-supported Christian militia in southern Lebanon, Maj. Saad Haddad yesterday withdrew it and resumed direction of the small portion of Lebanese border territory that his forces control.

In a radio broadcast in southern Lebanon, Haddad also said he had fired some officers in the eastern, predominantly Christian sector of the enclave, and that he was reorganizing the military command there.

During the previous two days, Israeli Army liaison officers had been urging Haddad to reconsider his resignation, and Haddad said he had also come under intense pressure from Lebanese civilians.

Washington Post correspondent William Claiborne also reported that the Israeli Cabinet formally adopted two conditions yesterday for any country's participation in the Sinai peace-keeping force.

In one condition, the Cabinet declared that the force, which will patrol the Sinai after Israeli withdrawal on April 25, will not supervise the pullback of Israeli forces. The British foreign secretary, Lord Carrington, has said Britain decided to contribute troops to the force because it wanted to ensure the return of Arab lands to the Arabs.

The Cabinet also voted to bar participation in the force by any nation that referred in public statements to the European Common Market's Venice Declaration, which spelled out the terms of a European Middle East peace initiative.

Foreign Minister Yitzhak Shamir, meanwhile, said that Western political and military support to the Arab oil powers "may cause us to reassess our attitudes" toward further concessions for peace. In a speech to the United Jewish Appeal, Shamir sharply condemned the Reagan administration's decision to sell radar planes to Saudi Arabia and its encouragement of a Saudi peace plan.