Secretary of State George P. Shultz quickened the pace of his talks with Israeli officials today in what U.S. sources said was an attempt to get an agreement by the weekend for the withdrawal of Israeli forces from Lebanon.

Details of the day-long meetings remained secret, and there was considerable public skepticism here about whether Shultz would be able to carry his shuttle mission to a successful conclusion that quickly.

One warning sign came from Israeli Prime Minister Menachem Begin. With Shultz standing by his side, Begin told reporters tonight, "There are still problems to be discussed here and in Beirut."

However, Israeli officials called the talks "extremely constructive" and said they had been conducted "in a friendly businesslike atmosphere." Among diplomatic negotiators, the use of terms like "constructive" and "businesslike" is generally understood as a sign of progress and optimism.

At issue in the negotiations are Israeli demands concerning security arrangements in southern Lebanon that the Lebanese government considers infringements on its sovereignty.

Despite the skepticism about an imminent agreement, senior U.S. officials accompanying Shultz privately sketched a "best case scenario" that would see the secretary finish his talks with the Israelis on Tuesday morning and then fly to Beirut with the outlines of a plan that would overcome the differences blocking an agreement.

If the main points are accepted by Lebanese President Amin Gemayel, the officials continued, another day or two probably would be required to clear up last-minute hitches.

Then, according to this scenario, Shultz would go to Damascus, probably on Friday, to discuss the tentative accord with Syrian President Hafez Assad. The Israelis have made clear that they will not carry out any agreement to pull their troops out of Lebanon unless Assad consents to the simultaneous withdrawal of Syrian forces and Palestine Liberation Organization units in Lebanon under Syrian protection.

Lebanese Foreign Minister Elie Salem traveled to Damascus Monday for a three-hour meeting with Assad on the withdrawal negotiations, news services reported from Damascus.

Assuming Assad does not reject the plan outright or raise objections requiring immediate attention, Shultz would then try to schedule stops in Jordan and Saudi Arabia to confer respectively with King Hussein and King Fahd. He then would go to Paris where he is scheduled to attend a meeting of the Organization for Economic Cooperation and Development beginning Monday.

But, while the United States would like to see things unfold according to that timetable, U.S. officials said it will require another day or so to determine whether it is, in Shultz's word, "doable."

The officials conceded that the negotiations could bog down and that Shultz might have to consider canceling his trip to the OECD meeting or returning to the Middle East after the meeting.

"The situation is very fluid, anything is possible," one senior U.S. official said. "It's impossible to predict what might happen."

In today's talks, conducted primarily with Foreign Minister Yitzhak Shamir and Defense Minister Moshe Arens, Shultz followed the same approach that he had used with Lebanese negotiators in Beirut on Saturday and Sunday. That was described by sources familiar with the negotiations as going through a tentative draft agreement line by line to isolate key points of disagreement.

As one U.S. official put it, "He knows what the Lebanese maximum positions, fallback positions and bottom lines are on each disputed point. Now he's getting the same things from the Israelis and trying out different formulations with them that might close the gaps. The aim is to enable him to go back to Beirut with language on all or most of these points that Gemayel will be able to buy."

However, the differences, at least as expressed publicly by both the Begin and Gemayel governments, are so pronounced that a big question remains about whether Shultz can overcome them through changes and rearrangements in wording.

Primarily at issue are security arrangements in southern Lebanon that the Lebanese contend are being interpreted by Israel in a way that will permit it to retain a residual military presence inside Lebanon. Despite vehement Lebanese objections, the Israelis also are insisting on a leading role in these security arrangements for their ally, the militia force of Saad Haddad, a former Lebanese Army officer.

The United States is known to agree with the Lebanese argument that Israel's demands would violate Lebanon's sovereignty and undermine the authority of the Gemayel government, giving Gemayel little room for further compromise.