The leader of one of the important Christian factions in Lebanon said he told White House officials yesterday that he believes Christian militia forces should be allowed to continue to operate in the war-torn country until Syrian troops and armed Palestinians have left.

Camille Chamoun, 82, an ardently pro-U.S. former president of Lebanon and symbol of the militancy of the country's Maronite Christians, also said he did not believe that Lebanon could sign a peace treaty with Israel because "it would mean that Lebanon would have to cut all its relations with the Arab world and be isolated from the Arab world and the Arab economy."

But Chamoun said he thought that the new Lebanese government would be willing to modify an existing security agreement with Israel, signed in 1948, to provide the assurances Israeli leaders are seeking. He said he had mentioned this idea informally to Israeli diplomats and U.S. Middle East envoy Morris Draper.

Lebanese Army regulars, backed by armor and accompanied by French and Italian troops, began conducting house-to-house searches for illegal aliens and arms dumps in Moslem West Beirut last week. But, according to local press reports in Lebanon, President Amin Gemayel has decided to delay any attempt to disarm Christian Phalangist militiamen in East Beirut until after his visit to the United States next week, when he is scheduled to see President Reagan.

The United States, anxious for the Lebanese Army to reestablish its authority throughout Lebanon, has called repeatedly for all independent armed militias in the country to be disbanded. A primary role of the multinational force is to provide security until the Lebanese Army is capable of exercising full authority.

The tall, silver-haired Chamoun was the head of the Christian Front during the civil war of 1975-76 and as president during a 1958 crisis he called in the U.S. Marines. He said he is friendly with Gemayel but his government had met opposition in Lebanon because of Gemayel's failure to put "known" figures in his Cabinet. "Personally, they are nice people but they lack experience and they lack prestige," he said.