United Nations took another painstaking step today toward independence for the territory of Namibia by announcing general agreement on a "multiparty meeting" in January linked to a March cease-fire, U.N.-supervised elections and independence by the end of next year.

In a cautiously worded report to the Security Council, Secretary General Kurt Waldheim noted that South Africa, which rules the territory, and the other parties involved, had given their "assent" to his proposals.

This assent means that for the first time South Africa has committed itself in principle to a specific date for the launching of the U.N. operation that will lead to independence for the territory, also known as Southwest Africa, which has been ruled by South Africa since a 1920 mandate from the League of Nations.

But both U.N. and American officials readily agreed that today's announcement leaves Pretoria with a large loophole to escape any final agreement.

South African acceptance of Waldheim's proposals was conditioned on the establishment of "trust and confidence" in the United Nations as an impartial mediator. This, Waldheim's report pointed out, is a "subjective and imprecise criterion." Any party, the report said, might "use it as a pretext for delay."

The U.N. operation, if it does get underway March 1 as now planned, will involve the stationing of 7,500 U.N. troops in the territory and in a 60-mile demilitarized zone on the border. The troops would monitor bases in Angola and Zambia used by the liberation movement, the Southwest African People's Organization.

The plan would include elections for a constituent assembly that would draw up a constitution for the new nation.

The January "pre-implementation" meeting under Waldheim's auspices is designed to dispel the "acute mutual mistrust," the report said. South Africa had asked for the all-party conference, but the United Nations, SWAPO, the African nations and the West had insisted that the meeting be linked to the March 1 starting date for the U.N. operation.

South Africa accepted this link on Friday, Waldheim said, thus clearing the way for today's proposal.

No venue is set for the conference, but it is expected to be held someplace in Africa between Jan. 7 and Jan. 15.

Western diplomats said South Africa had insisted that the meeting be delayed until just before the Reagan administration takes office in Washington to avoid the possibility -- rumored here -- that President Carter might go along with Security Council economic sanctions against Pretoria in the event that South Africa does not agree to the U.N. timetable.

The participants in the January meeting are expected to include five Western nations, including the United States, a number of African states, SWAPO and the political parties sponsored by South Africa inside Namibia. The South Africans would be involved, in some intentionally vague way, as "interlocutors."

The agonizing Western drive to push South Africa toward the granting of independence without actually facing the issue of sanctions has now gone on for more than four years, since South Africa refused U.N. guidelines to resolve the status of Namibia.