American officials confirmed yesterday that progress has been made in the Middle East summit conference, but they cautioned that there remain "substantial differences" on which the final success or failure of the talks now hinge.

The first official characterization of the discussions, now in their fourth day at nearby Camp David, was contained in a carefully worded, three sentence statement that had been cleared by the U.S., Egyptian and Israeli delegations before being relayed to reporters by White House press secretary Jody Powell.

"Progress does seem to hav been made in some areas," Powell said. "However, substantial differences remain on other important issues. There is simply no basis at this point for any informed speculation on the final outcome of this summit meeting."

The overall impression left by the end of the day was that U.S. officials were less optimistic than they had been earlier.

It was clear from Powell's comments that while some differences have been narrowed at the summit sessions of President Carter, Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin, the "progress" that has been made could easily evaporate if the remaining differences are not resolved.

". . . In many cases, one issue hinges upon another and so forth," Powell said. "I frankly want to warn you from looking at this as a process in which we just stack the pieces up. Unfortunately, it is not quite that simple."

In response to a question, Powell added that "it is not an unreasonable assessment" to presume that "the progress which seems to have been made on some issues may be dependent upon additional progress on other issues."

There was no hint of which issues appear closer to resolution and which remain major sticking points. Powell would say only that his characterization of "progress" on some and "substantial differences" on others related to "important areas of issues."

Beyond these few carefully crafted descriptions, what was actually going on inside the secluded presidential retreat six miles west of here remained, as it has from the first, excruciatingly murky.

The pace of the summit slackened yesterday as Begin observed the Jewish Sabbath. No meetings were held among the three leaders, and direct, formal negotiations among them may not resume until tomorrow.

Powell said there has been "no deadline set either formally or informally, for the conclusion of this conference," which is expected to last until Tuesday or Wednesday.

"It is my assessment that the parties, all three parties are interested in continuing as long as they feel that there is some constructive purpose to be served by doing so," he said.

While Begin observed the Sabbath, Carter and Sadat were said to be "reviewing the situation at this point" separately with their advisers.

Carter met for an hour yesterday morning with Secretary of State Cyrus R. Vance and National Security Affairs Adviser Zbigniew Brzezinski Sadat, according to Powell conferred with his principal advisers later in the day.

Friday night, the president and Rosalynn Carter spent two hours at dinner with the prime minister and Aliza Begin. It was the first time since the summit began that the president has shared a meal with either of the other two leaders and it fit into a pattern of the summit that has seen considerably more American-Israeli contact than meetings between the U.S. delegation and the Egyptians.

It has been widely assumed here that the early spate of U.S. - Israeli meetins involved U.S. pressure on Begin for compromise, particularly on his refusal thus far to consider an Israeli withdrawal from the West Bank of the Jordan River, one of the key issues in the Middle East dispute.

At this point in the summit, however, Sadat may be feeling growing pressure as well as he considers his next moves in what has been described as "serious discussions" with Begin.

According to Egyptian officials, Sadat arrived at Camp David prepared to push for a total failure of the talks rather than accept half-way measures that might only further weaken his standing in the Arab world. Given Powell's description of the talks as now hinging on the remaining "substantial differences," Sadat is likely nearing a crucial juncture at which he must decide whether to make further concessions on those differences or force an outright break-down in the negotiations.