United States Air Force pilots were involved in a top-secret rescue mission that brought about 700 Ethiopian Jews out of the Sudan Friday, administration officials and Jewish sources revealed yesterday.

"They took out everybody who was there," said one official, "but the numbers were off" in regard to how many of the refugees, fleeing famine in their own country, would be waiting for the airlift alongside a gravel strip near the Sudanese town of Gedaref.

"It was a humanitarian operation," the official said. The Air Force C-130 transport planes originated from Frankfurt, Germany, officials said, and flew the refugees from the Sudan via the Red Sea to Israel.

Administration officials said Sudanese President Jaafar Nimeri approved of the airlift on condition that it be kept secret. The details were worked out in a meeting between Vice President Bush and Nimeri on March 6.

The White House, State Department and Pentagon refused yesterday to comment on the airlift. Informed sources said one of Nimeri's conditions for allowing the black Ethiopian Jews, called Falashas, to be flown out was that nothing be said either in the United States or Israel about the operation. The Israeli state-run radio and television networks carried no reports of the airlift.

Nimeri is to fly to Washington on a private visit for a medical checkup Wednesday and is to meet with President Reagan and other U.S. officials later in the week.

Administration sources declined yesterday to give the specific number of Falashas flown out of Sudan. But sources said the Air Force crews expected two or three times the number that were located.

An official of the American Association for Ethiopian Jews (AAEJ), Howard Lenhoff, said he had learned from sources in Israel that "close to 700" Falashas had arrived safely there.

He said the number of Falashas previously thought to be awaiting the airlift in Sudanese refugee camps was 1,500; he was unable to account for the discrepancy.

"Where are the others, I don't know," Lenhoff said in a telephone interview from his home in Costa Mesa, Calif. He said he understood that "a couple" of the C130s had arrived in Israel empty. Lenhoff was the immediate past president of the AAEJ and serves as its director of research.

In February, AAEJ officials estimated that there were 3,000 to 4,000 Falashas in Sudan, but they subsequently revised the number to 1,500 based on reports from immigrants arriving in Israel.

Conditions in the refugee camps were reported in February to have deteriorated, and at one point AAEJ officials were receiving reports that as many as 25 Falashas were dying a day. But State Department officials dispute accounts that as many as 1,200 Falashas died of famine and disease after suspension in January of an Israeli airlift. It was not clear whether the deteriorating camp conditions accelerated U.S. and Israeli plans to airlift the remaining Falashas out of Sudan.

The Los Angeles Times, in a dispatch filed from Gedaref Saturday, said 10 propeller-driven C130s landed at a gravel airstrip eight miles north of the town to pick up refugees who had been in the Tawawa refugee camp.

The planes, according to the dispatch, came in at dawn in an operation conducted in tight secrecy and planned by the Central Intelligence Agency.

Israel airlifted about 7,800 Falashas from the Sudan to Israel between Nov. 21 and Jan. 6 in Operation Moses. The airlift was called off after news of it leaked out.

Although the airlift officially ended in early January, Falashas apparently continued to arrive in Israel by other secret means. Lenhoff said he thought that possibly as many as 800 had left secretly and that this might explain why the American pilots found fewer Falashas than expected.

Lenhoff said the AAEJ's main concern was the fate of the estimated 8,000 Falashas remaining in Ethiopia out of the 26,000 who had been officially counted by an Israeli-sponsored group in 1976.

The Israelis believe that the Falashas are a "lost" Jewish tribe that migrated centuries ago to Ethiopia, where they long have been discriminated against because they owned no land.

Since the onset of the Ethiopian revolution in 1974, efforts have been under way to move the entire Falasha population out of Ethiopia to Israel. There are now about 16,000 living there.

The few administration officials who would discuss the airlift yesterday indicated that it was a one-time operation.