The newly elected South African-backed leaders inNamibia conditionally agreed today to U.N. supervised elections in the territory prior to its independence next year.

Their decision included the provisio that the elections take place on a fixed date before Sept. 30. It also stipulated that there should be no reduction in the South African troops stationed in the Pretoria-administered territory until there is a total cessation of hostilities in the guerrilla war between South Africa and the black nationalist guerrilla movement, the Southwest Africa Peoples Organization(SWAPO).

The action raises hopes that implementation of the Western-inspired U.N. election plan can be resumed. The plan was delayed for almost three months following Pretoria's decision Sept. 20 to hold unilateral elections earlier this month for local leaders in Namibia (also known as Southwest Africa).

South African Prime Minister Pieter W. Botha said he has advised the newly-elected constituent assembly to accept the Western-backed U.N. blueprint for indepedence for this strife-torn territory.

Both told a news conference in the Nambian captial Windhoek, "We are prepared to have another election here. we have advised the elected representatives that they should accept it."

South Africa has run the territory since 1920 undet a League of Nations mandate revoked by the United Nations in 1966. The five Western members of the U.N. Security Council-the United States, Britain, West Germany, France and Canada-put together the plan for U.N.supervised elections and a transition to independent black-majority rule in the territory.

Dirk Mudge, chairman of the Democratic Turnhalle Alliance which dominates the 50-man constituent assembley chosen by that election, said today that other unresolved problems, such as the size and composition of the U.N. peacekeeping force should be handled by the South African-appointed adminstration general of the territory, Marthinus Steyn.

Mudge told the assembly that there are "serious discrepancies" in the U.N. plan. Among them are the international body's recognition of SWAPC as the sole legitimate representative of the Namibian people. Mudge today asked that the recognition be revoked but did not make it a condition with the U.N. plan, as he had been hinting privately he would do.

Mudge's preconditions regarding the U.N. plan are unlikely to be acceptable to U.N. Secretary General Kurt Waldheim or Sam Nujoma, leader of the SWAPO guerrillas.

A SWAPO spokesman characterized the constituent assembley's action today as "a smokescreen and a pathetic attempt at political escapism." He said its "smbiguous and vague stand" on the U.N. plan demonstrated its fear of competing with SWAPO in U.N. supervised elections.

Meanwhile, South African police in Windhoek announced th release of three top SWAPO leaders who live inside Namibia.They include Deputy Chairman Daniel Tjongarero, Transport Secretary Solomon Gamathand youth leader Johannes Konjore. They had been detained without charges since Dec. 2 after a march protesting this month's elections.