More than 20 Finnish soldiers from the United Nations force stationed in southern Lebanon were detained and reportedly threatened with death by the Israeli-backed South Lebanon Army today in a bizarre incident that underscored the continuing instability in Israel's so-called "security zone" north of the Israeli-Lebanese border.

Timur Goksel, spokesman for the United Nations Interim Force in Lebanon (UNIFIL), said that 22 U.N. soldiers were still held in two locations late tonight by South Lebanon Army militiamen who were demanding the release of 11 of their comrades who either were captured by or defected to a rival militia in the area earlier today.

The South Lebanon Army, in a statement released through the Israeli military command in Tel Aviv tonight, charged that Finnish troops from the U.N. force attacked one of their units and disarmed the militiamen before turning them over to the Shiite Moslem militia Amal.

The Israeli Army appeared to support this version of the incident. The military spokesman said the Israeli Army's Northern Command had agreed to help gain the release of the captured U.N. soldiers, but only if UNIFIL acted to free the 11 South Lebanon Army militiamen from Amal.

In a separate incident, armed Shiite Moslems forced the Austrian ambassador to Lebanon and the Canadian chief of a U.N. aid agency into the besieged camp of Burj al Barajinah on the outskirts of Beirut today, special correspondent Nora Boustany reported from Beirut. The armed Shiites demanded help in negotiating the release of relatives trapped inside the Palestinian camp. The diplomats were allowed to leave three hours later.

Goksel said U.N. troops had surrounded the village of Qantara, where three of the Finnish soldiers were being held and threatened with death by a South Lebanon Army force of about 50 militiamen. He said that four "Israeli personnel," apparently advisers to the South Lebanon Army, also were in the village.

Nineteen other Finnish soldiers were held on a bus in the Lebanese border village of Adaisse, where they were stopped by the South Lebanon Army on their way back from Israel, Goksel said.

In New York, U.N. Secretary General Javier Perez de Cuellar appealed to the visiting Israeli defense minister, Yitzhak Rabin, to help win the release of the Finnish soldiers.

Goksel said it was not clear how the 11 Lebanese militiamen fell into the hands of Amal, but added, "I am sure that the Finnish soldiers did not turn them over to Amal." Asked about the charge that the Finnish troops attacked the South Lebanon Army, Goksel said, "That is not the way the U.N. works, and everybody, even the Israelis, knows it."

Israeli radio quoted the U.N. military command as saying the South Lebanon Army was threatening to kill a U.N. soldier every hour unless their 11 comrades were released by Amal, which has vowed to drive the Israeli-backed militia out of the area. The Israeli military spokesman said he had no information on the reported death threats or the U.N. assertion that four Israeli advisers were with the South Lebanon Army unit in Qantara.

Goksel said UNIFIL was "in contact with the Israelis at all levels" and was seeking Israel's intervention to end the incident. "We are waiting to hear from them," he said.

The incident in southern Lebanon highlighted the problems Israel continues to face in the border security zone, where the South Lebanon Army, with Israeli Army backing, is supposed to maintain order.

Israel had planned to announce by this week that it had withdrawn all of its forces from Lebanon, three years after the 1982 Israeli invasion. While no official explanation has been offered here, the failure to meet that target apparently was linked to growing doubts about the capability of the South Lebanon Army, which is trained, equipped and financed by Israel.

The 5,000-strong UNIFIL force was sent to southern Lebanon by a Security Council resolution in 1978, following Israel's first invasion of the territory. There has been almost constant friction between the U.N. units and Israel's proxy militia forces in the area.

Goksel said the incident began at about noon, when a South Lebanon Army unit took over a U.N. checkpoint at Qantara and accused the three Finnish soldiers manning it of turning over their 11 comrades to Amal. He said two Finnish officers were sent to the village to investigate. They also were detained but later were released.

Meanwhile, he said, another South Lebanon Army unit stopped a U.N. bus carrying 19 Finnish soldiers back from leave in Israel. He said the U.N. command was in radio contact with the bus but had no soldiers in the vicinity, an area near the Israeli border where Israel never has allowed U.N. units to operate.

Special correspondent Boustany reported from Beirut:

Peter Gallagher, 55, the director of the U.N. Relief and Works Agency (UNRWA), which specializes in aid programs for Palestinian refugees, and Austrian Ambassador Georg Znidaric, 43, were leading a convoy of trucks carrying food and water into the Burj al Barajinah camp when they were stopped by an armed group of five women and one man.

The armed group ordered the diplomats to seek the release of seven Shiite Moslems held hostage by Palestinian guerrillas defending the camp. When the diplomats replied that UNRWA could not get involved in such negotiations, the gunman fired shots under their car and threatened to hurl a hand grenade into it. Three hours later, Amal leader Nabih Berri radioed a guarantee that the Austrian and UNRWA officials could leave, and they emerged from the camp unharmed.