The main rightist Christian militia leader in Lebanon has pledged in principle to sever his Phalangist forces' controversial links with Israel as part of a formula designed to end the fighting that has battered this country for six years, Phalange Party sources said today.

Syria's demand for an end to the Christian militias' relations with Israel has been a major stumbling block for Arab mediators working, in parallel with U.S. special envoy Philip C. Habib, to devise a plan for Lebanon that would staunch the bloodshed here and, it is hoped, ease the Syrian-Israeli crisis over Syrian SA6 antiaircraft missiles in eastern Lebanon.

It was not immediately apparent whether the reported Phalange statement -- the terms of which remained secret -- is specific and far-reaching enough to satisfy Syrian demands. But it was greeted as a breakthrough by Lebanese government officials quoted in the Beirut press today.

Habib is scheduled to return to the Middle East shortly and the reported Phalangist concession is likely to play a role in his deliberations. Syria has refused to negotiate in detail on a pullout of its missiles or other peacemaking measures as long as the Lebanese Christians continue to receive military and poltical support from Israel, dealings that in Syrian eyes are tantamount to creation of a fifth column along Syria's southwestern flank.

Deliberations of an Arab committee -- comprising Syrian Foreign Minister Abdul Halim Khaddam; the Saudi and Kuwaiti foreign ministers, Prince Saud and Sabah Ahmed Sabah; as well as Arab League Secretary General Chedli Klibi -- ended abruptly and inconclusively Sunday when the Phalange Party failed to come up with the statement on Israel.

Lebanese Premier Shafiq Wazzan said today, after President Elias Sarkis received the Phalangist statement, that he hopes the Arab foreign ministers will accept an invitation to convene again earlier than July 25, the date of their deferred meeting.

Phalange sources said the 33-year old Christian militia commander, Beshir Gemayel, visited Sarkis last night and submitted to him a "comprehensive statement rejecting in principle relations with Israel."

Gemayel and his father Pierre, the Phalange Party head who met with Sarkis today, both publicly stated they are ready to "take any stand which would facilitate the mission of the four-member committee and make of it a success."

But they declined to reveal exactly what was in the statement given to Sarkis. The younger Gemayel said he put the declaration in the hands of the president to use it whenever he deems it appropriate.

The official Syrian radio ignored the development, which dominated the Lebanese media today, and concentrated on announcing the downing of a pilotless Israeli drone over the Bekaa Valley.

The Israeli Army said the drone crashed because of a malfunction. There was no official reaction to the reports from Beirut on the Phalange vow to cut links with Israel.