Two Israeli officers and four guerrillas were killed in a clash last night near eastern Lebanon's tense Bekaa Valley, and Israel and Syria each expressed concern today that the other was preparing for increased hostilities there.

In another sign of difficulty for Lebanese peace efforts, Israeli and Lebanese negotiators failed in six hours of talks to make progress on terms for an Israeli withdrawal from Lebanon. A week ago Israeli officials had reported increased optimism over the negotiations.

An Israeli military spokesman said that the fighting erupted near Sofar on the strategic Beirut-to-Damascus highway when Israeli forces acted to stop "terrorists" from penetrating their lines. He did not identify the nationality of the guerrillas, who may have been from Palestine Liberation Organization forces stationed in Syrian-controlled areas of the northern Bekaa.

In Jerusalem, Israeli officials blamed the officers' deaths on Syria, saying that Damascus could control guerrilla infiltration, Reuter reported. But an Israeli Foreign Ministry spokesman said that his country did not want a conflict with Syria.

Israeli newspapers said that Israeli forces have been put on an increased state of alert in response to stepped-up Syrian military activities, United Press International reported. The reports apparently were based on a statement Wednesday by Defense Minister Moshe Arens to the Foreign Affairs and Defense Committee of the Israeli parliament.

Syria's state-run radio accused Israel of reinforcing its troops in the Bekaa and of staging "provocative" military exercises in the Golan Heights, which Israel captured from Syria in the 1967 Middle East war and later annexed. Syria also repeated earlier charges that an Israeli attack on Syria is imminent.

The fears expressed by Israel and Syria came amid a dispute over a recent assurance by President Reagan to Syria that a comprehensive Middle East peace settlement would include considering return of the Golan Heights to Syrian control.

In a rebuff to Reagan, Israeli Prime Minister Menachem Begin on Wednesday said that the annexation of the Golan Heights was irreversible, United Press International reported. "The law, the jurisdiction and the administration of the state of Israel is in force and will remain in force on the Golan Heights," Begin said.

The clash near the Bekaa was noteworthy because most Israeli soldiers killed in Lebanon in recent months have been in areas of southern Lebanon distant from Syrian lines. In the most recent of these attacks, an Israeli soldier was killed yesterday on the coastal highway in an ambush of the vehicle in which he was traveling. There have been so many attacks on the road, which links Beirut and Israeli-controlled towns to the south, that some Israeli soldiers call it the "Ho Chi Minh Trail."

After the latest round of Lebanese-Israeli negotiations in the Beirut suburb of Khaldah, a Lebanese spokesman said that the talks were difficult and had made no progress. An Israeli spokesman said the problems were related more to the wording of a draft accord than to disputes over principle.

The differences apparently are over Israel's desire to retain some form of military presence in southern Lebanon after withdrawal and over the role of former Lebanese Army major Saad Haddad, who commands an Israeli-allied militia in the area.

Israeli officials had said in Jerusalem a week ago that the two sides were very close to agreement in the talks and that the negotiations were not expected to take too much longer. The optimism was viewed in part as the result of a lessening of pressure on Israel following the refusal of Jordan to enter talks under the auspices of President Reagan's Middle East peace plan, which Israel has rejected. Since then, however, senior Israeli officials have said that the bombing of the U.S. Embassy in Beirut helps to justify Israel's insistence on obtaining adequate security guarantees before withdrawing.

U.S. marines and rescue workers continued to pull bodies from the embassy's rubble. An embassy statement said that 15 Americans were known to have been killed in the explosion and that two others were still missing and presumed to be dead. In addition, 17 Lebanese employes of the embassy were confirmed dead and 13 others were missing and presumed dead, the statement said.

Lebanese security sources said that 49 bodies had been pulled from the embassy's concrete debris. That figure apparently includes passers-by and others killed in the explosion Monday, and they said that the final death toll was expected to exceed 60.

The capital awaited the arrival of a U.S. delegation headed by Undersecretary of State for Political Affairs Lawrence Eagleburger to escort home Saturday the bodies of the dead Americans.

The edginess here was heightened briefly this morning when gunfire erupted near a forward position of U.S. marines guarding the embassy ruins. There was some relief when it was later discovered that the bullets were from the rifle of a Lebanese Army soldier who fired at the car of a speeding motorist. He missed.

Reuter quoted Lebanese judicial sources as saying that five people detained in connection with the explosion had been released after investigators decided that they were not involved.