An Islamic group holding four kidnaped Soviet Embassy officials distributed photographs of the captives with cocked guns to their heads today and threatened to kill one of them tonight unless a Syrian-backed offensive against Moslem fundamentalists in Tripoli was halted.

The release of the demand and the photos by a previously unknown group calling itself the Islamic Liberation Organization came after conflicting reports about whether the Soviets, kidnaped from two automobiles yesterday, had been killed.

Telephoned reports early today had said two of the Soviets had been "executed" by their captors. But later a Soviet source here said information had been "conveyed to the Soviet Embassy by Syrian military officers in charge of security in Beirut and in contact with the abductors" that "all the hostages are alive and are in Beirut."

By late tonight there was no indication that any of the Soviets had been killed, although one of the statements by the alleged captors said the first would be killed by 9 p.m. local time if their demands were not met.

There also was confusion about who had abducted the Soviets and who was holding them. Callers yesterday had claimed that they were kidnaped by Islamic Jihad, a terrorist group that has claimed responsibility for several bombings against U.S. and other western targets and has said it is holding six Americans and four Frenchmen.

The Islamic Liberation Organization statement accompanying the photos today, however, disclaimed any connection with Islamic Jihad and added, "We also wish to note that there is no coordination between us and the Islamic Jihad organization." The Islamic Liberation Organization also distributed photocopies of the identification papers of two of the missing Soviets to local newspapers.

The Soviet Embassy officially confirmed the abductions for the first time today, identifying those kidnaped as press attache Oleg Spirin, commercial section representative Valery Mirikov, consular secretary Arkady Katkov and Dr. Nikolai Svirsky, the embassy physician.

The Soviet statement added that "for the time being the venue of their detention is not known. Measures are being taken and a search is under way."

In Moscow, the Soviet news media, in its first mention of the kidnaping, branded it a "heinous crime" and said that "all necessary steps" would be taken to save the captives. A statement by the official Tass news agency, read on state television, blamed "bandits from one of the archreactionary ultraright-wing organizations" in Beirut, Washington Post correspondent Celestine Bohlen reported.

The brief statement on the evening news made no mention of the captives' identities, the demands, or an execution deadline. The stern response to the taking of Soviet hostages was in contrast to Soviet press reaction last June to the seizure of Americans aboard a passenger plane at Beirut airport. Then, while condemning terrorism in general, Soviet spokesmen indicated that U.S. policy in Lebanon had precipated the crisis.

A Soviet source close to the embassy here said today that Moscow had launched "very high-level contacts with Damascus through diplomatic channels. You can be sure Mr. Gorbachev knows exactly what is going on."

Soviet charge d'affaires Yuri Suslikov flew by helicopter to meet with Lebanese President Amin Gemayel and remained in close contact with officials of the mainstream Shiite Moslem Amal militia, the Druze Progressive Socialist Party and Lebanese police, Soviet sources said.

The kidnapings, the first of any Soviet officials here, have sent shockwaves through the small Soviet community and brought strict directives to Soviets to stay close to the embassy or inside it.

When told in a call from a journalist about the photographs and the statement of the Islamic Liberation Organization, a Soviet source pleaded that copies of the pictures be sent over to the embassy for identification. "It is very dangerous for us to move about," he said.

One Soviet source, however, said it was unlikely the Soviets would phase out their presence. There are about 150 Soviet nationals in Beirut, including 50 embassy personnel.

The statements distributed by the Islamic Liberation Organization condemned the two-week-old campaign by Syrian-backed leftist Moslem forces against the Tawheed militia of fundamentalist Sunni Moslems in Tripoli, in northern Lebanon, and demanded that the Soviet Union, as Syria's chief arms supplier, pressure Damascus to bring an end to the campaign.

"Stop the advance on Moslem Tripoli and effect the retreat of atheistic forces from around this heroic city," one statement said. "All these forces and Syria must assume responsibility for the lives" of the Soviets. "We shall execute them all and also strike in strength."

A later statement said, "We will start carrying out the death sentence on the first hostage at 9 p.m. sharp, unless the atheist onslaught against Islamic Tripoli is stopped."

According to reports from Tripoli, the offensive by the Syrian-backed Lebanese leftist militias against Tawheed continued today.

The Voice of the Homeland, a private radio station there, quoted a Tawheed spokesman as saying "suicide counterattacks were being launched against the attackers to beat off the leftist offensive."

In apparently related attacks in Moslem west Beirut, several rockets hit offices and homes of officials of the Damascus-linked National Syrian Social Party, which wants a merger of Lebanon, Syria, Jordan and what was once Palestine. The attacks were carried out by underground Sunni Moslem groups.

Soviet influence against Syria or the captors of its diplomats here appeared to be no stronger than that of the United States in similar situations.

One Soviet analyst here commented that "in these circumstances, the Soviets will try to pressure Syria through diplomacy, but I don't know what the outcome will be."

He noted that the Soviets had sent Syrian President Hafez Assad a personal message at the beginning of the Syrian-supported Shiite Moslem campaign against Palestinian refugee camps in Beirut, "but the war continued for weeks. Our advice to stop it fell on deaf ears. It is very difficult to talk to President Hafez Assad, who carries out his own policies in Lebanon."

The source close to the Soviet embassy said, "At present, Syria is our sole friend in the Middle East, because we have lost Egypt. It is not part of our policy to exert pressure on anyone."

The two countries signed a friendship and cooperation pact in 1980 and there reportedly are more than 5,000 Soviet military advisers in Syria.