TYRE, LEBANON, FEB. 18 -- Around-the-clock searches by U.N. peace-keeping troops and Shiite Moslem militiamen in the muddy countryside of southern Lebanon turned up no sign today of U.S. Marine Lt. Col. William R. Higgins, the head of a U.N. observer group who was kidnaped by gunmen near here yesterday.

An anonymous caller in Beirut claimed responsibility for the kidnaping on behalf of a previously unknown group called the Islamic Revolutionary Brigades, but authorities were unsure of its authenticity and most suspicions pointed toward one of the area's many militant, pro-Iranian factions.

The caller, in a telephone conversation with an international news agency in Beirut, accused Higgins of being "one of the directors of the CIA in southern Lebanon." He said the "hostage will not be freed until after his trial," and said the captors would follow up with another statement accompanied by a picture of the missing American -- the method other groups have used to prove that they are holding a captive.

Higgins, 43, who heads the 75-man Lebanon Group of the U.N. Truce Supervision Organization, was seized by gunmen and taken to an unknown destination yesterday as he was driving by himself, behind another U.N. car, enroute from Tyre to U.N. troop headquarters in Naqurah, near the Israeli border.

Abdel Majeed Saleh, the political commander of the Shiite Amal movement in southern Lebanon, who had met here with Higgins and his aides for 1 1/2 hours just before he was intercepted, said the American was probably "the captive of the same people holding other foreign hostages in Lebanon." Most are being held by pro-Iranian factions.

Speaking at his home here, four miles north of Ras Ain, where Higgins was ambushed, Saleh said that there appeared to be a "determination to destroy an international sphere of influence in Lebanon and its humanitarian organizations for the sake of swapping any foreigner within reach for prisoners abroad."

Although he did not name the Iranian-sponsored Hezbollah, Saleh clearly was referring to their hostage-taking strategy in Lebanon.

Hezbollah's clergy and followers appeared nervous and edgy in the Tyre area today and largely kept a low profile. Some were seen racing through U.N. checkpoints on the coastal road just north of Tyre, refusing to stop.

Timor Goksel, spokesman for the U.N. peace-keeping force, said in a telephone interview that there were no leads as to where Higgins had been taken.

Shiite Amal militiamen, in coordination with U.N. troops, checked cars and expanded a 24-hour hunt for Higgins in rural villages. Amal fighters said they were under "strict orders" to find Higgins and not to sleep before locating him.

Amal officials said fog, rain and poor visibility had helped the kidnapers and prevented effective searches by U.N. helicopters.

U.N. Undersecretary General Marrack Goulding cut short a visit to Damascus and flew back to Beirut today upon hearing of Higgins' disappearance. He met with Lebanese President Amin Gemayel and later condemned the kidnaping.

"Here is an American who came to Lebanon to take part in an enterprise set up to assist the withdrawal of Israeli troops from Lebanon, to support Lebanese sovereignty and Lebanese state institutions," Goulding said.

"When you have a situation in which three international staff have been taken hostage within two weeks, that is a very serious situation," he added, referring to the abduction two weeks ago of two senior Scandinavian officials with the U.N. Relief and Works Agency in Sidon.

In New York, U.N. spokesman Mario Zamorano said that the purpose of Goulding's visit to Gemayel was to "stress the need for this matter to be resolved as speedily as possible." He said Goulding drew the Lebanese government's attention "to the possible implications for United Nations activities in Lebanon of the fact that three United Nations officials have been kidnaped during the last two weeks" -- a statement seen in New York and here as a warning that U.N. agencies may further cut back their activities here.

"We don't know who did it," Goulding said. "We do not know why they did it. But what I can say is that these incidents do obviously have an effect on the readiness of governments to send their people whether as military or civilians to work in Lebanon."

Aly Yassin, a fundamentalist Shiite religious leader in Tyre, speculated, however, that Higgins had been singled out for kidnaping primarily because he was an American, not because he was a member of a U.N.-affiliated organization.

Goksel said Higgins had believed he was safe traveling in this area of southern Lebanon because he was the chief of his unit. Other Americans in his group are largely confined to U.N. headquarters in Naqurah, near the Israeli border.

Diplomatic sources in Beirut said Higgins was considered to be somewhat of a "cowboy" and appeared overly confident in his venture to travel to the Tyre area.

Saleh said Higgins' visit here was his second in three months.

Goksel said there had been no previous threats or indications that Higgins was in any danger as a U.N. officer. He was wearing his U.S. Marine Corps uniform along with the U.N. insignia and blue beret at the time of his abduction.