The 21 Finnish soldiers from the United Nations peace-keeping force in southern Lebanon who have been held hostage for a week by an Israeli-backed militia will be freed on Saturday, the Israeli military command announced tonight.

The announcement followed a day of high-level discussions here and public statements by a senior U.N. official that military force would be considered if the 21 Finns were not released quickly.

A U.N. spokesman confirmed the Israeli Army announcement, saying that the release of the soldiers, members of the U.N. Interim Force in Lebanon (UNIFIL), was scheduled for 11 a.m. (4 a.m. EDT) Saturday in the southern Lebanese town of Marjayoun, where the hostages are being held. Marjayoun is the headquarters of the Israeli-backed South Lebanon Army.

The apparent resolution of the dispute, which deepened the longstanding animosity between Israel and UNIFIL, came a few hours after Brian Urquhart, the United Nation's undersecretary general, expressed his exasperation with the situation in an unusual, on-the-record meeting with reporters here.

Urquhart, who met today with senior Israeli government officials, emphasized that the use of U.N. force would be a "last resort," but he repeatedly returned to the possibility of such an action.

"We cannot have peace-keeping forces kept as hostages by groups of this kind," he said.

Urquhart, a veteran diplomat, also obliquely but clearly criticized Israel, which until tonight had supported the South Lebanon Army's demands and claimed that it had no control over the militia, which is trained, equipped and financed by the Israeli Army.

"If there is no other control over the South Lebanon Army, we will have to consider other options . . . I am not making threats. It is merely a fact. We can't accept a situation like this," Urquhart said.

The hostage dispute was triggered June 7, when 11 South Lebanon Army militiamen, all of whom are Shiite Moslems, turned up in the Lebanese port city of Tyre. Tyre is under the control of the South Lebanon Army's rival, the Shiite Moslem militia Amal.

In what it described as a retaliatory move, the South Lebanon Army then captured more than 20 Finnish soldiers from UNIFIL, charging that the Finnish U.N. battalion had attacked and disarmed one of its units and turned over the 11 militiamen to Amal.

The commander of the South Lebanon Army, retired Brig. Gen. Antoine Lahad, who had the public support of Israeli Defense Minister Yitzhak Rabin and other senior officials here, demanded the return of his 11 men in exchange for the release of the captive Finns.

The breakthrough in the hostage dispute followed a meeting earlier today between officials of the International Committee of the Red Cross and Lahad. The Red Cross officials told Lahad that the 11 militiamen said in interviews yesterday that they do not wish to return to the South Lebanon Army.

The militiamen were interviewed individually yesterday by Red Cross and U.N. officials in the southern Lebanese village of Maarake, the headquarters of the French battalion of UNIFIL. "The 11, of their own free will, left the South Lebanon Army and don't want to rejoin it," Urquhart said.

Israeli officials said the Red Cross representatives also delivered letters to Lahad from the 11 militiamen confirming that they had defected to Amal.

A senior Israeli official said Israel put no pressure on Lahad to release the 21 captive Finns. "He was convinced that once they signed those letters there was very little he could do," the official said.

U.N. officials have maintained from the outset that the militiamen deserted to Amal, although there have been suggestions that some of the Finnish UNIFIL soldiers may have exceeded their authority by facilitating the transport of the militiamen to Amal leaders in Tyre.

Israel has invested heavily in the South Lebanon Army, which is designed to be the Israelis' main surrogate force in a "security zone" just north of the Israeli-Lebanese border. A senior Israeli official said tonight that Israel's refusal to pressure Lahad to end the hostage dispute earlier resulted from Lahad's pleas to his sponsors.