U.S. officials believe the chances for an Israeli-Syrian agreement on withdrawal of Israeli forces from Lebanon have improved, but they caution that an accord is not likely for several months and that there is little the United States can do to speed up the process.

That, the officials said yesterday, is their assessment of the impressions gained by Richard W. Murphy, assistant secretary of state for Mideast affairs, during four weeks of diplomatic explorations in major Middle East capitals. Murphy, who was in Europe yesterday, is expected to return here this weekend.

Although Murphy's trip had several objectives, his main aim was to explore further the possibilities for an active U.S. mediating role in persuading Israel, Lebanon and Syria to agree on a formula for ending Israel's occupation of southern Lebanon.

The officials, who declined to be identified, said Murphy found the gap between the contending parties too wide to offer any hope of immediate progress toward a breakthrough. For that reason, the officials continued, the Reagan administration, which was stung badly in its earlier attempt to influence events in Lebanon, intends to maintain a low profile and refuse, for now, to be drawn formally and visibly into a mediating process.

But, the officials also said, Murphy did get the impression that Syrian President Hafez Assad's government, which holds veto power over the actions of the Lebanese government, has not rejected definitively the withdrawal terms proposed by Israel last month.

For that reason, the officials said, Murphy, a former ambassador to Syria who has a rapport with Assad, is cautiously optimistic about the eventual chances for an agreement that would see the Israelis pull out in exchange for Syrian promises not to move their forces south of their current positions in Lebanon, to prevent the return of Palestine Liberation Organization guerrillas to southern Lebanon and to permit the deployment of U.N. troops into areas vacated by Israel.

That assessment is known to be shared by a number of Arab diplomatic sources familiar with the Syrian government's thinking. When Israel's new prime minister, Shimon Peres, visited here last month, senior sources accompanying him said they believed Syria would find a basis for interest in the Israeli withdrawal proposals because they are modeled on past truce agreements between the two countries that were successful.

Specifically, the Israeli thinking is based on the fact that Assad, despite his bitter enmity toward Israel, never has violated the so-called "red lines" established by past disengagement agreements to separate Syrian and Israeli forces in areas such as the Golan Heights. As a result, the Israelis say they believe Syria can be trusted to adhere to new "red-line" arrangements in Lebanon.

The withdrawal plan announced by Israel on Oct. 17 had four conditions: a Syrian commitment not to occupy those areas of southern Lebanon vacated by the Israeli army, a similar commitment to prevent guerrilla activity in those areas, continued deployment of the Israeli-sponsored South Lebanon Army along the Israeli-Lebanese border, and the stationing of U.N. troops in a zone north of the South Lebanon Army that would stretch from the Lebanese coast in the west to the Syrian border in the east.

So far, neither Syria nor Lebanon, which takes its cue from Damascus, has given any outward sign of accepting these conditions. Instead, U.S. officials said, Murphy and other diplomats have the impression that Assad intends to stand pat through the winter, when difficult weather and terrain will make the Israeli forces more vulnerable to Lebanese terrorist attacks.

Syria's apparent aim, they said, is to see whether Israel's mounting economic difficulties and domestic pressure for withdrawal will weaken the Peres government's resolve and force it to make more concessions.

But, the officials continued, Murphy came away with the impression that if Israel stands firm, Syria is likely to accommodate at least those elements of the Israeli proposal creating new "red lines" and using U.N. forces as a buffer in the vacated areas. The sources added that Murphy found far greater Syrian reluctance to accept continuance of the South Lebanon Army as a force independent of the Lebanese government.

But, the officials stressed, before such fundamental issues can be addressed, a great deal of preliminary manuevering and posturing can be expected in the negotiations, which technically are between Israel and Lebanon and which already have been hampered by Israel's detention of Shiite Moslem leaders in southern Lebanon and Lebanon's demand that Israel pay $10 billion in war reparations.