BEIRUT, FEB. 20 -- A clampdown on Moslem fundamentalist groups backed by Iran spread from southern Lebanon to the western Bekaa Valley today as Shiite militiamen pressed their search for a kidnaped American officer attached to a U.N. observer force.

Leaders of the mainstream Shiite Amal militia hinted that Marine Lt. Col. William Higgins, who was kidnaped Wednesday in southern Lebanon, may have been seized in an effort by supporters of Iran to reactivate talks on an exchange of hostages for weapons needed by Iran.

An official of the Palestine Liberation Organization said the PLO also had evidence to suggest Higgins was kidnaped for use in a potential deal with the United States. "We believe Iran wants to make a new deal with the Americans using Higgins," said PLO official Salah Khalaf in an interview with Reuters news agency in Tunisia.

In Beirut, a senior member of Amal's politburo, Mohammed Beidoun, said he suspected that "the basic aim behind Higgins' abduction was a pressure campaign . . . concerning hostages and weapons."

As the search for Higgins resumed, Amal members stormed strongholds of pro-Iranian extremists in southern Lebanon for the fourth straight day.

Tension between the moderate Amal and radical Hezbollah (Party of God) mounted in the western Bekaa as the rival militias set up checkpoints near the town of Machghara, Shiite officials said tonight.

There was still no trace of Higgins, the chief of the Lebanon unit of the U.N. Truce Observation Supervision Organization (UNTSO), abducted by gunmen south of Tyre. His abduction has been claimed by two groups, the previously unknown Islamic Revolutionary Brigades and the Organization of the Oppressed on Earth, which also claimed responsibility for the June 1985 hijacking to Beirut of a TWA airliner.

Using two helicopters and police dogs, dozens of troops of the U.N. Interim Force in Lebanon (UNIFIL) combed the countryside about eight miles southeast of Tyre.

UNIFIL troops and Amal militiamen focused their search on a number of rural towns in that area after receiving tips from local informants. An Amal commander said the militia last night entered the southern village of Jibsheet, a Hezbollah stronghold, and rounded up suspects after a standoff with some of its armed residents.

Amal officials said a number of Iranian Revolutionary Guards had also been taken in for questioning.

Justice Minister Nabih Berri, who is the leader of Amal, ridiculed allegations that Higgins was a CIA agent. In a television interview, he said Higgins "may be an intelligence agent {for the CIA} and he may not. But . . . a lot of westerners kidnaped before were accused of being spies. Some of them, for example some Frenchmen, were later released. Did they stop being spies? What kind of pretext is this?"

A communique by the Organization of the Oppressed on Earth, delivered yesterday to the offices of an international news agency, rejected American peace efforts in the region and the dispatch of U.S. envoys here. Secretary of State George P. Shultz is expected in the region in the next 10 days.