U.N. officials began an investigation today into the origins of the deadlocked, four-day-old hostage dispute between the U.N. peace-keeping force in southern Lebanon and the Israeli-backed South Lebanon Army (SLA).

French Brig. Gen. Jean Pons, the deputy commander of the U.N. Interim Force in Lebanon (UNIFIL), was placed in charge of the investigation into the incident, in which more than 20 Finnish soldiers from the U.N. force were abducted Friday by the SLA.

The militia, with the backing of its Israeli sponsors, still holds 21 of the Finns, demanding that the rival Shiite Moslem militia Amal release 11 SLA militiamen who were captured last Friday.

Both Israel and the SLA have charged that a Finnish U.N. unit assisted Amal in the capture of the 11 militiamen.

U.N. and Finnish officials have said the militiamen, all Shiite Moslems, deserted from the SLA but it has not been made clear why they were turned over to Amal.

Timur Goksel, the U.N. spokesman, said U.N. officials were informed today that the 21 Finnish captives have been moved to the southern Lebanese town of Marjayun, where the headquarters of the SLA is located.

He said U.N. liaison officers met with an SLA official today in an attempt gain access to the Finnish soldiers by U.N. medical teams but that no agreement was reached because the militia is demanding a meeting with the 11 men being held by Amal.

"The problem is that they have our men but we don't have their men," he said.

As the hostage dispute has dragged on, it has become increasingly clear that Israel hopes to use the incident to gain concessions from the U.N. force for de facto recognition of the SLA, which is trained, equipped and financed by Israel. The militia is designed to be the main security force in a "security zone" six to 15 miles wide that Israel has established just north of the Israeli-Lebanese border and that extends into territory policed by U.N. units.

Israeli officials said today that the willingness of U.N. officers to meet with an SLA representative already represented "de facto recognition" of the militia. They also asserted that the U.N. decision to investigate the incident was an "admission that something was wrong" in the capture of the 11 militiamen by Amal.

Israeli Defense Minister Yitzhak Rabin charged yesterday that the U.N. force has shown favoritism toward Amal, at the expense of the SLA, in its dealings with the rival militias.

In a radio interview this morning, Goksel dismissed Rabin's charge as "the most ridiculous statement I've heard" on the hostage incident.

Goksel said the U.N. force is authorized to recognize and deal officially only with the Lebanese government, but because of the chaotic situation in southern Lebanon it has been forced to establish informal contacts with the area's various armed militias.