The body of a Soviet diplomat, shot in the head at close range, was found this morning dumped in a wasteland south of Beirut after a caller claiming to speak for Moslem fundamentalist kidnapers told a western news agency where to look for it.

The caller, saying he spoke for the Islamic Liberation Organization, threatened that the three remaining Soviet hostages kidnaped here Monday afternoon would be killed "one after the other if the atheistic campaign against Islamic Tripoli does not stop."

Syria, Moscow's closest ally in the Middle East, has backed an offensive by leftist forces in the northern city of Tripoli against strongholds of the Sunni Moslem fundamentalist militia known as Tawheed. The fighting, which began Sept. 15, has caused at least 200 deaths and has led to the evacuation of many of the city's 700,000 residents.

The killing of the Soviet diplomat, identified as consular secretary Arkady Katkov, was followed by another call to an American news agency in the name of the previously unknown Islamic Liberation Organization, warning that the Soviet Embassy in west Beirut would be "demolished" over the heads of "Soviet diplomatic staff and members of the KGB," the Soviet secret police. The caller set a deadline of 48 hours for the Soviet compound to be evacuated.

Soviet Embassy officials said there would be no general evacuation of embassy personnel, but other sources raised the possibility that nonessential personnel would be withdrawn. Soviet sources close to the embassy expressed fears that "a hunt for Soviets has now begun. The situation is very, very serious for all of us."

Although the authenticity of the threat against the embassy could not be verified, security around the well-guarded compound just off the Corniche Mazraa was tightened. One source close to the embassy said the warning was "being taken seriously. We have taken security precautions around the embassy with the help of the Progressive Socialist Party, our friends."

Trucks of the party's Soviet-equipped Druze militia, armed with heavy machine guns, were moved into defensive positions around the embassy.

A Soviet source said that embassy officials were "astounded by the assassination" of Katkov. "Yesterday," the source said, "the Syrians guaranteed the safety of the four hostages. They said everything was normal."

In Paris, where Soviet leader Mikhail Gorbachev arrived for an official visit, Kremlin spokesman Leonid Zamyatin told a press briefing that the Soviets have made a "request and a plea" to Syria and to various Lebanese factions to obtain the release of the remaining three Soviet hostages. Zamyatin said that Moscow never puts pressure on other countries but, he said, "we have taken a series of diplomatic steps" and "addressed a plea" to several parties for help.

The Soviet spokesman said that Gorbachev had "devoted the major part of his day" to the hostage situation on Tuesday, and is "doing everything he can" to "save lives, to protect the Soviet citizens in Lebanon," Washington Post correspondent Gary Lee reported.

The Soviet Union believes that the act of terrorism demonstrated by the execution of one of the hostages is "a gross violation of all standards of international law," Zamyatin said. The Soviet Union, he added, always opposes "any act of vandalism, whether it affects Americans or any other citizens of the world."

In Moscow, a government statement read on the main evening news program announced the killing of one hostage but did not identify Katkov as the victim. The statement called the killing "an atrocity that cannot be pardoned," Reuter reported.

Soviet sources here said that Gorbachev had sent Syrian President Hafez Assad a letter yesterday. "You can see what the response has been," one source close to the embassy said. "It does not depend on Hafez Assad or Gorbachev but on this band of gunmen holding the hostages."

Two other Soviet diplomats, commercial representative Valery Mirikov, 37, and Oleg Spirine, 32, a press attache, and embassy physician Nikolai Svirsky were abducted at gunpoint in two separate incidents Monday in the first such operation targeting Soviet personnel in Beirut. Katkov, 32, was married and had one child.

"These hostages are not the last," said one Soviet source here, who was among the officials who identified Katkov's body at the morgue of the American University Hospital. "What can we do?" he asked. "What would the Americans have done?"

Reporters who saw Katkov's body said he had a gunshot wound in his upper left cheek and coagulated blood around his mouth, nose and chin. Chief Coroner Ahmed Harati, who examined the body, said Katkov had been shot in the temple at a range of "no more than a few centimeters" with a 7-mm automatic weapon.

One Soviet resident here, a friend of Katkov, reacted emotionally to the news of his death. "We have lost a person," he said, crying. "We must do something to save the other citizens and friends."

Officials at the Soviet Embassy, who had dealt calmly with the press until this morning, lost their composure on receiving police confirmation that Katkov's body had been found.

"Questions, questions, questions!" yelled one embassy official, trembling. "This is not a time for questions. You must have some kind of moral conscience. What is important is that the hostages are set free. There has been nothing but nonsense on the radios."

Beirut radio stations have been broadcasting a string of contradictory communiques made on behalf of Islamic Jihad, a separate group from the Islamic Liberation Organization. According to these broadcasts, two Soviet diplomats had been killed yesterday. One news agency, reacting to a police report that another body had been found, announced today that a second Soviet had been executed. The body turned out to be that of a 20-year-old Lebanese man.

The weekly flight to Beirut of the Soviet Aeroflot airline was canceled today, due to "the security situation," sources close to the Soviet Embassy said. They said that if an evacuation of Soviet dependents were ordered in the next few days, a special plane would be chartered from Moscow for them.

The fate of the three remaining Soviet hostages appeared tonight to hinge on events in Tripoli, where the Syrian-backed leftist forces were increasing the pressure on the fundamentalist Tawheed, or Islamic Unification Movement.

There were indications here that the leftist militias, with Syria's tacit approval, would not accept a cease-fire as demanded by the kidnapers here unless three conditions were met. These were democratic freedom in Tripoli, where daily life has been ruled by the fundamentalists according to strict Islamic regulations since 1983; the collection of arms from all factions, and the entry of Syrian troops into the city, Lebanon's second largest.

The Tawheed leader, Sheik Saeed Shaaban, traveled to Damascus last night during a truce of several hours for meetings with Syrian Vice President Abdul Halim Khaddam and other officials. The fundamentalist cleric was accompanied to Damascus by an Iranian team and by the spokesman of the Iranian-backed Hezbollah, or Party of God, Sheik Ibrahim Amin.

Shaaban is believed to be resisting the deployment of Syrian troops in Tripoli, which the Syrians and their allies are demanding as a way to end the fighting. Syria is determined to control Tripoli to guard against a return to the city by forces loyal to Palestine Liberation Organization Chairman Yasser Arafat.

Syrian intelligence officials and journalists who toured the approaches to Tripoli this morning reported that Shaaban's Tawheed forces are blockaded from all sides. The leftist forces -- members of the Arab Democratic Party, the pro-Syrian Baath Party, the National Syrian Social Party and the Lebanese Communist Party -- claim to have captured about 30 buildings in one strategic quarter called Abu Samra, where diehard Tawheed fighters are dug in.

Although fighting was continuing, the heavy artillery bombardment of the city from the Syrian-controlled hills around it subsided.

A statement issued here tonight in the name of the Islamic Liberation Organization said, "We declare readiness to set free the three hostages should the truce continue, a total cease-fire prevail, armed men be withdrawn and a pledge be made that attacks on Moslem Tripoli do not resume."

The statement warned that if the kidnapers' demands were not met, "We will continue to execute the hostages and will hold the Soviet Union responsible for . . . its stooges in Lebanon."

President Amin Gemayel expressed his regrets and condolences at Katkov's killing to Soviet Charge d'Affaires Yuri Souslikov. The Shiite Moslem Amal and Druze militias expressed "shock" and "deep sympathies" at "this cowardly action."