A caller claiming to represent the Islamic Jihad terrorist organization warned today that U.S. and French captives held by the group would be harmed if western or Arab governments pressure Syria to obtain their release.

The message, which could not be authenticated, also said that some of the captives had been "liquidated," but it gave no details. It comes against a background of speculation that Syria, in an effort to deflect international suspicion that it is involved in terrorist operations, may be pressing Islamic Jihad to release the captives.

Although expressing "respect and appreciation" for Syrian President Hafez Assad, the message, the first attributed to Islamic Jihad since March 13, also appeared to be a warning to Damascus not to take drastic action against Moslem fundamentalists suspected of holding the captives.

"We know of pressures by several countries, including two Arab states, on President Assad," the statement, read over the telephone to an international news agency, said. "We warn all these countries, whether America, France or any other Arab country involved in these pressures, that we will make the earth shake under their feet because we will resort to our destructive methods if they don't stop the maneuvers against Syria and Islamic Jihad."

It added that if those countries "don't stop their maneuvers against Syria and Islamic Jihad, the first to pay the price would be the remaining American and French hostages we have."

Without elaborating, the caller claimed that the number of captives Islamic Jihad was holding "has decreased," and he said, "We have liquidated a number of them."

There was no indication whether the caller was referring to previously asserted killings. On March 5, a caller claiming to speak for the pro-Iranian Islamic Jihad said the group had killed French sociologist Michel Seurat. Last Oct. 4, a caller said that U.S. diplomat William Buckley, kidnaped in 1984, had been killed in retaliation for an Israeli raid against Palestine Liberation Organization headquarters in Tunisia. No body has been found in either case, however.

A message said to be from Islamic Jihad was delivered to the British-owned television news agency Visnews on March 13 with a videotape of three of the French captives. That message said that all future communiques from Islamic Jihad would be typed and accompanied by photographs or other evidence to avoid confusion over false claims.

There was nothing delivered with today's call and no way to verify its authenticity.

The caller also said that Islamic Jihad was not the same as the Iranian-supported guerrilla group Hezbollah, as many observers have suggested.

Syrian troops in Lebanon's Bekaa Valley have clashed at least twice this month with Hezbollah members and their supporters, amid rumors that Damascus was ready to crack down on Moslem fundamentalists in Lebanon.

A widening American focus on the sources of terrorism in the Middle East has prompted press speculation that Syria might attempt to dispel international suspicion of its involvement in such operations by gaining the release of the captives. A leftist Beirut newspaper, As Safir, quoted Syrian and Iranian sources last week in an article suggesting that there had been a breakthrough in stalled negotiations over the captives.

Islamic Jihad claims to be holding Associated Press bureau chief Terry Anderson; David Jacobsen, the director of the American University hospital; the Rev. Lawrence Jenco, Beirut director of Catholic Relief Services, and Thomas Sutherland, a dean at the American University of Beirut, as well as three Frenchmen -- diplomats Marcel Carton and Marcel Fontaine and journalist Jean-Paul Kauffmann.

Worsening economic conditions in Syria and its dependence on Iranian oil supplies make it awkward for Damascus to defy Tehran's wishes on treatment of the fundamentalists Iran backs in Lebanon, according to analysts here.

Syria, meanwhile, has experienced its own domestic turmoil. On April 20, it disclosed that a number of people were killed and wounded in a series of explosions aboard several intercity buses on April 16. Security officials initially blamed the attacks on Israeli agents operating out of Lebanon.

But later, three Syrians and two Turks were charged with setting off the bombs, and they said on Syrian state television a week ago that they were members of the banned Moslem Brotherhood. The suspects confessed on the air to having received instructions and money from Iraq, whose government has been at odds with that of Syria, but Iraqi Information Minister Latif Nassif Jassim denied that his country was involved in the bombings.