The Security Council today extended the life of the 7,000-man U.N. peace-keeping force in southern Lebanon for two months and in the process reaffirmed its earlier cease-fire resolutions, which had demanded the immediate and unconditional withdrawal of Israeli troops from the country.

The United States voted for the resolution, but American officials argued that the council action did not mean that the Israeli withdrawal must be either immediate or unconditional.

The U.S. position is that an Israeli pullout should be linked with the withdrawal of Syrian troops from Lebanon, the establishment of a strong national government there and adequate security arrangements in the area of the Israeli border.

Earlier this week, U.S. diplomats had told their colleagues on the council that because the situation had changed since the adoption of the original resolutions on June 5 and 6, Washington could no longer accept a reaffirmation of unconditional withdrawal.

The vote on the resolution was 13-0, with the Soviet Union and Poland abstaining, as they traditionally have done on the periodic renewals of the Lebanon peace-keeping force.

The irony of the situation is that the U.N. troops have no real military function as a buffer force now that Israelis have overrun their positions.

But all the parties to the dispute remain anxious to keep the force in place until a final arrangement can be worked out, "to preserve in place the capacity of the U.N. to assist in the restoration of the peace," as the resolution said.

Some diplomats suggested that the two-month extension provides a deadline of sorts for negotiation of the ultimate security arrangement, now being undertaken by American representative Philip C. Habib.

The resolution also authorizes the U.N. troops to take on the new task of "extending their protection and humanitarian assistance to the population of the area." In fact, they have done this on an impromptu basis since the fighting began, providing shelter, medical care and food to civilians in the Tyre vicinity after overcoming objections by Israeli commanders.

The Israelis have objected to this formal enlargement of the U.N. role. U.S. Ambassador Jeane J. Kirkpatrick told the council that, in Washington's view, the resolution permits no "extension of the responsibilities, functions or territorial scope" of the U.N. troops.