Cardinal John O'Connor of New York arrived here today on what he called a mission of support from the Roman Catholic Church in the United States and offered to assist in efforts to gain the release of Americans and others held captive by Shiite Moslem militants.

O'Connor, who said he had spoken with the families of the American captives before coming here, met with President Amin Gemayel immediately upon his arrival. Later he told reporters that he had offered to do whatever he could to secure the hostages' freedom, including visiting Syria or Moslem-controlled west Beirut.

O'Connor's arrival coincided with the announcement of a Syrian-arranged cease-fire that at least temporarily has ended a four-week-old battle between the Shiite Amal militiamen and Palestinian guerrillas at the Palestinian refugee camps south of Beirut. But new clashes broke out between pro-Syrian militiamen and pro-Iranian Shiite forces in the Bekaa Valley today.

At least nine persons died and 40 were wounded in the Bekaa Valley fighting, centered in the town of Mashgara. The two militias there battled house-to-house, and shelling was reported. Beirut Radio said Syrian troops and Amal militiamen had intervened to enforce a truce there, wire services reported.

O'Connor, who is president of the Catholic Near East Welfare Association a humanitarian organization active here, said he would not interfere in any negotiations already under way to free the hostages. In an apparent effort not to raise hopes about a possible negotiating role on behalf of the captives, he told reporters that he considered his three-day visit "purely pastoral."

U.S. Embassy officials said it was their understanding that the main motivation for the visit was the expression of "church solidarity."

Five Americans are still missing in Lebanon, including a Roman Catholic priest, the Rev. Lawrence Jenco of Catholic Relief Services. The others are Associated Press Beirut bureau chief Terry Anderson, Thomas M. Sutherland and David P. Jacobsen of the American University of Beirut, and U.S. Embassy political officer William Buckley, who is believed to have been killed by his captors, although no body has been found.

Anglican Church envoy Terry Waite, a lay representative of the Archbishop of Canterbury, has made several unsuccessful trips to Beirut to negotiate the release of hostages.

"Naturally, I'll do whatever I can to assist," O'Connor said. " His trip to Lebanon is part of a church initiative sponsored by the Lebanese Maronite patriarchate, the papal nuncio and the pontifical mission, a Vatican-affiliated organization that oversees charity services and church contributions.

"I told the president that I hope that soon there will be some breakthrough in the question of our American hostages. I have spoken with their families. Their families wanted me to convey their love and prayers," he added.

O'Connor said he was encouraged by Lebanese expatriates and clergymen in the United States to visit Lebanon as a gesture of support. With him, he said, he carried a message of assurance that "the church in the U.S. has not forgotten the people of Lebanon."

Palestinian officials contacted inside the refugee camps tonight expressed doubts that today's cease-fire, which was marred by occasional machine-gun fire, would be a lasting one. Fighting between Moslem groups has left 150 people dead and 800 wounded.

Statements by Amal and the pro-Syrian Palestine National Salvation Front, which opposes Palestine Liberation Organization chairman Yasser Arafat, set the truce at 6 p.m. local time and said the two groups would supervise its implementation.

According to a plan hammered out in Damascus with Syrian Vice President Abdul Halim Khaddam, the Lebanese Army and police force would take over Amal positions surrounding the refugees camps of Sabra, Shatila and Burj al Barajinah, where a fierce battle between Palestinian guerrillas and Amal militiamen for control of the camps has raged for more than a month.

Palestinian sources said prospects for the cease-fire were dim because not all Palestinian factions were adequately consulted in the Damascus deliberations.

The Shiite Amal militia, the dominant paramilitary group in west Beirut, is opposed to a drive to rearm Palestinian guerrillas in Lebanon and wants to confine them to their shantytowns. Palestinians argue they must be armed to defend civilians living there and to fight Israel in southern Lebanon.