The commander of the Israeli-supported South Lebanon Army said today that his forces will be able to secure all of southern Lebanon in a year, but that a complete Israeli withdrawal will be impossible unless there is also a political agreement to prevent outside pressures on his army.

Brig. Gen. Antoine Lahad gave this assessment of his army's progress, of which even some Israeli military officials are skeptical, in an interview at the headquarters building he shares here with units of the Israeli Army.

Lahad said that for the South Lebanon Army to accomplish the Israeli-assigned task of policing southern Lebanon, it will have to double its current strength of about 2,200 men. He said this can be accomplished in a year, and the army would then be ready to deal with any local security problems.

But if "political pressure" originating outside of southern Lebanon takes the form of armed intervention in the area, "the South Lebanon Army may not be strong enough" to deal with it, Lahad said.

"I personally think there can be no complete withdrawal of the Israeli Army from south Lebanon before there is a political agreement" to prevent such intervention, he added.

Lahad did not specify what political groups and military forces most concern him. But he said they were all "manipulated by Syria," making Syrian agreement to the operations of the South Lebanon Army essential, in this view, to the army's survival and the prospects of an Israeli withdrawal.

Lahad's comments underscored the new emphasis being given by Israeli officials to the importance of obtaining security guarantees from Syria as the key element in achieving an Israeli troop pullout.

Both Syria and the Lebanese government in Beirut have objected to any role in southern Lebanon for the Israeli-trained and financed South Lebanon Army, which they view as merely Israeli proxies. The Israelis, however, insist that they see no alternative.

Lahad, 55, a Maronite Christian, took command of the army last spring, after the death of Maj. Saad Haddad, who commanded an Israeli-supported militia in far southern Lebanon that was the forerunner of the South Lebanon Army.

A few months ago Lahad was saying publicly that it would take two years for his army to be ready to take over security duties throughout Israeli-occupied southern Lebanon. He gave no reasons today for believing the job could be done in less time, but his more optimistic projection appears closer to the timetable of the new Israeli government, which has vowed to achieve an early Israeli withdrawal.

According to Israeli Army figures, the South Lebanon Army has grown by only about 200 men since June, although about 150 new recruits are said to be trained every five weeks. The ethnic makeup of the force has also remained at more than 60 percent Christian, with about 18 percent of the soldiers Shiite Moslems and the others Druze or Sunni Moslems.

A western source who is familiar with the South Lebanon Army put the size of the force at 1,600, about 600 less than claimed by Lahad, and said the Israelis "can trust only about 500 of them," mainly Christian soldiers originally recruited by Haddad.

Shiite Moslems are the majority population of southern Lebanon and attracting more Shiites to the South Lebanon Army is considered by all an essential element of success.

Lahad, who spoke through an interpreter from behind a desk decorated with a small Lebanese flag and a photograph of Haddad, said southern Lebanese Shiites are gradually overcoming "fear of reprisals" and now make up more than half of his recruits. But he and Israeli military officials acknowledged concern that the upsurge in Shiite recruiting may be due in part to an attempt by Amal, the Shiite Moslem political and military organization in Lebanon, to infiltrate the South Lebanon Army.

Senior Israeli officers said a handful of infiltrators has been detected and expelled.

Lahad also said there had been "no side effects" from a recent incident in the Shiite village of Sohmer in which 13 people were shot to death by Druze soldiers from the South Lebanon Army in retaliation for an earlier ambush near the village. Lahad said such incidents may occur in the future and suggested that the Sohmer shootings had been blown out of proportion.

"These things may happen," he said. "The people of Lebanon do not give them the same importance as the western media . . . we cannot separate the South Lebanon Army from its human element."

Lahad said seven Druze soldiers had been detained in connection with the shootings, but they had not been brought to trial yet for fear of interfering with an attempt to arrange a "reconciliation" between Sohmer and Rashaiya, a nearby Druze village and home of the soldiers.

According to a senior Israeli officer in southern Lebanon, Lahad's men now control about 50 percent of the territory occupied by Israel and are responsible for security in those areas. He said one of the most hopeful recent signs was the South Lebanon Army's successful takeover on Sept. 4 of Nabatiyah, a largely Shiite city of 50,000 and a center of anti-Israeli activity.

The officer said the only successful Israeli withdrawal will have to follow this pattern of a gradual pullback in stages to see if the South Lebanon Army can handle its increasing responsibility. He called Lahad's force "an improvisation," but said he saw no prospect "for years" of the regular Lebanese Army taking up positions in the south to prevent guerrilla infiltration into the area.

"We need to test, and for that you need time," he said.