Lebanese army forces are expected to move into southern Lebanon "within days" to bring the sensitive area near the Israeli border under government control for the first time since 1975, Lebanese Foreign Minister Fuad Butros said yesterday.

As soon as the army re-establishes control, Butros said, the border with Israel will be closed, ending a 16-month policy by which Israel allowed thousands of Lebanese to cross the border for commerce, employment and medical care.

Butros indicated, however, that the Lebanese government, still torn by two years of factional strife and civil war, would probably take no steps against those residents of the south, most of them Christian, who cooperated openly with Israeli troops and authorities.

"The moment the south is under control of the Lebanese government, however, there will no longer be any reason for further contact with Israel and the border will be closed," he said in an interview here.

Israel initially opened the border in June 1976 to allow Lebanese, cut off by the civil war from normal medical facilities to the north, to come into Israel for treatment at mobile clinics.

In an effort to win support of rightist Christian residents of the area. Israel then allowed Lebanese to cross the border for a wide range of activities. Hundreds took jobs in Israel and others sold produce and handicrafts there.

As the contacts broadened, Israel increased its military cooperation with Lebanese Christian forces in the border region and, this year, sent troops into the area to help the rightists fight the Moslem leftist and Palestinian forces.

A cease-fire negotiated with the assistance of U.S. officials took effect Sept. 26 and, under an agreement reached earlier among Lebanon, Syria and the Palestine Liberation Organization, Lebanese army troops are to gradually replace Palestinian commandos in the area near the Israeli border.

At least two battalions of Lebanon's army, which collapsed and virtually vanished during the civil war, have been rebuilt and re-equipped to the point where they can maintain control of southern Lebanon "if all parties meet their obligations" under the cease-fire, Butros said.

Lebanon has asked the United States for $50 million in military assistance for the current fiscal year and a Lebanese army team is in the United States now working out details of the request, Butros, who is also defense minister, said.

Butros, who met with President Carter and other U.S. officials in New York Wednesday, made it clear that the U.S. role in arranging the cease-fire in southern Lebanon had been significant.

He said that after his talks with Carter, he was optimistic that "Israel will not interfere any more" in southern Lebanon and he indicated that Israeli agreement to remain out of the area was directly connected to the overall American-Israeli accord announced yesterday on Middle east peace objectives.

"In my opinion, especially in the present context, it is not in the interest of Israel to interfere," he said.