Assistant Secretary of State Richard W. Murphy, signaling increased U.S. involvement in the negotiations for withdrawal of Israeli forces from southern Lebanon, will return to the Middle East this week in a new attempt to break the stalemate in the Israeli-Lebanese talks.

U.S. officials said Murphy tentatively plans to spend about two weeks in the area. His latest trip comes less than a month after he completed a four-week "fact-finding visit" that involved considerable shuttling between Mideast capitals to help get the negotiations launched.

The Reagan administration, which was embarrassed by the scuttling of the 1983 Israeli-Lebanese peace agreement mediated by Secretary of State George P. Shultz, publicly has refused to characterize its latest efforts as formal mediation. The official U.S. position is that the parties in the dispute are too far apart to justify the United States' taking such a role.

But, as U.S. officials privately acknowledged yesterday, Israel, Lebanon and Syria, which holds veto power over actions by the Lebanese government, all regard the United States as the only power that can overcome the maneuvering and posturing impeding the negotiations.

The officials insisted that the administration has no intention now of naming a new high-level special Mideast negotiator or even of using the term "mediation" in connection with Murphy's efforts.

Still, despite determination to keep as low a profile as possible, it is now clear that Murphy has assumed at least some of the functions of a special negotiator and is likely to spend considerable time in the weeks ahead trying to resolve the Lebanon withdrawal problem.

In particular, diplomatic sources said, there are a number of issues -- among them the disposition of U.N. forces after an Israeli withdrawal and the status of the Israeli-backed South Lebanon Army -- that are too sensitive to be settled by the low-level Israeli and Lebanese delegations conducting the talks. It has become clear, the sources said, that the negotiations need someone like Murphy who is able to shuttle between Jerusalem, Beirut and Damascus sounding out areas of compromise on these questions.

Murphy, who is assistant secretary for Mideast affairs, is known to have returned from his earlier trip cautiously optimistic that Syrian President Hafez Assad eventually will permit the Lebanese to agree to a plan that would interpose U.N. forces into areas vacated by Israel.

But Murphy and other U.S. officials also are understood to believe that Syria wants to drag out the talks in hopes of extracting further Israeli concessions.

On the other side, Israel reportedly is prepared, if the talks fail to make progress in the next two months, to withdraw its forces unilaterally to new positions along the Litani River that would leave an Israeli-held buffer zone extending 15 miles into Lebanon.