The U.N. Security Council has unanimously renewed the mandate of the U.N. peace force in southern Lebanon for only three months, rather than the usual six, after France told members it would withdraw its 783-member logistics unit from the operation if the longer term were approved.

The Soviet Union voted for the resolution Friday night for the first time since the U.N. Interim Force in Lebanon was created in 1978. Until now, Moscow had abstained each time the issue arose.

Soviet Ambassador Yuri Dubinin said his government would now start paying its annual $18 million share of UNIFIL's costs, but recognized no obligation to repay the $152 million it withheld over the past eight years.

The force, now consisting of 5,825 members, has been plagued since its inception by an inability to deploy its units along the Israel-Lebanon border. Despite the Soviet decision, it still faces financial difficulties because of the withholding by the United States of $21 million, 15 percent of UNIFIL's budget.

French Ambassador Claude de Kemoularia said the objective of the short renewal time is to increase pressure on all parties to resolve the problems facing the operation, both financially and militarily.

The new French prime minister, Jacques Chirac, had promised during his election campaign to lower France's profile in Lebanon, reducing the vulnerability of its nationals there, and recently withdrew French observers from the Beirut area.

The French won their point in the council, despite the stated preference for a six-month extension by Lebanon, the United States, the other eight countries supplying contingents to UNIFIL and the secretary general.

Secretary General Javier Perez de Cuellar warned in a report to the council that if UNIFIL were to be withdrawn, "there would be an immediate escalation of fighting in southern Lebanon, leading to "an increase in attacks against Israel and to an escalation of military action by Israel against Lebanon. A further major crisis could easily result."

Today's action postponed that eventuality by at least three months. However, American officials cautioned that no results were likely on Capitol Hill within that time on an administration request to restore full U.S. funding for UNIFIL. The American share of its budget -- $42 million, or 30 percent -- was cut back to $21 million for fiscal 1986 by Congress.

American officials expressed hope that an apparent softening of past Israeli displeasure with UNIFIL would make it easier for Congress to restore full funding.

On Capitol Hill, staff members of the Senate Appropriations subcommittee chaired by Sen. Warren B. Rudman (R-N.H.) said there was "still significant opposition" from members to full funding, and there would be no consideration of the issue for several months.

In practical terms, U.N. officials, American diplomats and the congressional staffers agreed that an increase in funding is unlikely unless Israel gives a clearer signal of its approval.

The Israeli view is that UNIFIL serves to a minimal extent as a barrier to guerrilla infiltrators and poses a problem by shielding them from Israeli strikes and by providing unwelcome publicity about such raids.

Any agreement to let UNIFIL redeploy to the frontier, in what is now Israel's self-proclaimed security belt, would have to satisfy Jerusalem's need for security guarantees. So far the United Nations has not been able to provide these.