BEIRUT, Lebanon (AP) _ Rocket and artillery barrages blasted Beirut on Monday after Christian army commander Michel Aoun rejected a proposed committee to monitor a cease-fire in his 6-month-old war against Syrian troops.

Police said the exchanges at times averaged 20 shells a minute and that at least six people were killed and 14 wounded.

That brought the toll to 921 killed and 2,713 wounded since March 8, when the confrontation began between Gen. Aoun's estimated 20,000 troops and Syria's 40,000 soldiers.

Concentrated shelling of the Shiite Moslem slum of Haret Hreik demolished a seven-story apartment building. A police spokesman said three bodies were found in the rubble of the Mokdad building, seven people were wounded and three more were missing and feared dead under the debris. He said the building collapsed while its inhabitants were waiting out the shelling in the basement.

Rescue workers hanging from ropes pulled a bloodied woman from the basement. She apparently survived.

The spokesman, speaking on condition of anonymity, said the duels broke out around 1 a.m., 90 minutes hours after Aoun announced in a televised interview his rejection of a security committee proposed by Arab mediators Saturday. He wants at least one Syrian committee member.

''I won't accept at all a Lebanese-Lebanese committee. This is what the Syrians wanted to deceive the world and say the clashes have been among Lebanese factions,'' Aoun said.

In Damascus, the Syrian capital, U.S. Ambassador Edward Djerdjian urged the Lebanese ''to demonstrate flexibility.''

The British Foreign Office expressed support for the Arab initiative and said ''an immediate and comprehensive cease-fire throughout Lebanon'' should be the first step.

And in an extraordinary burst of optimism, the Arab League's ambassador to the United Nations, Clovis Maksoud, said on Monday that Lebanon's long nightmare ''is on the brink of resolution.''

Arab League envoy Lakhdar al-Ibrahimi of Algeria, the Arab mediators' choice to lead the proposed committee, met with Aoun for an hour at the general's bomb-ravaged presidential palace in suburban Baabda.

He said later, ''We discussed ways of implementing the Arab committee's decisions.''

Why Aoun's gunners concentrated on Haret Hreik was not explained. The densely populated slum is a stronghold of the Iranian-backed Hezbollah, or Party of God. The Syrians have no firing positions there.

A bulldozer sifted part of the rubble, enabling rescuers with flashlights to climb down on ropes into the basement of the Mokdad building.

One worker shouted: ''A survivor 3/8 A survivor 3/8'' Rescuers rushed in with a stretcher and a blanket and pulled through a hole an unconscious woman covered with blood.

''She's alive 3/8 Allah akbar (God is great) 3/8,'' rescuers shouted as they put her into an ambulance.

The Arab committee - made up of King Fahd of Saudi Arabia, King Hassan II of Morocco and President Chadli Bendjedid of Algeria - announced a peace plan Saturday.

In calls for a cease-fire and a security committee and urges Parliament to meet on Sept. 30 to discuss political reforms, the Moslems' condition for ending the 14-year-old civil war.

The Syrian-backed Moslems want to share power equally with the Christians, who have dominated the government, legislature, army and judiciary since independence from France in 1943.

The Christians then were the majority, but Moslems now comprise 55 percent of the 4 million population.

On Sunday night, Aoun ruled out talk of political reform until Lebanon is ''free of all occupations.''

He was referring to Syria's 40,000 troops - in Lebanon under a 1976 Arab League peacekeeping mandate - and an Israeli force, which has controlled a border strip in south Lebanon since 1985.

U.S. Ambassador Djerdjian told reporters in Damascus:

''There's no way out of the Lebanese civil war except through a political solution, and the withdrawal of all foreign forces, including Israel's.

''The Arab League proposal is worthy of the support of the Lebanese people,'' he said, indirectly criticizing Aoun's reservations.

Aoun has accused the United States of turning a blind eye to Syria's war against the traditionally pro-Western Christians.

Washington withdrew its embassy staff from Christian east Beirut this month after Aoun's supporters besieged the compound demanding U.S. support.