The State Department gave an optimistic assessment yesterday of progress in the four-year-old negotiations for independence of Namibia, with a senior official saying that "if things go well" implementation of an agreement could begin in the South African-controlled territory before the end of the year.

Next week, negotiations will move to a new phase involving the role of the United Nations in the transition period to independence, according to the official, who under the ground rules of the press briefing could not be identified.

The numerous parties to the tortuous talks will thus bypass disagreement over a complicated election proposal and return to it later.

Chester Crocker, assistant secretary of state for African affairs, will meet this weekend and next week at the United Nations with a number of African foreign ministers in an attempt to speed the process, which the official said had reached a "critical phase."

The second phase has long been regarded as the most difficult. It involves withdrawal of the more than 20,000 South African troops fighting guerrillas in the territory, and installation of a U.N. peacekeeping force to supervise an election for an assembly to draw up a constitution.

Nevertheless, the official said "if things move according to the kind of timetable we have in mind it is foreseeable that we could within the next several months be in a position where the parties can agree on a date for implementation and that date could be in the next few months as well."

Such optimism that a settlement was near, however, has frequently been heard during the low-key 15 -year-old war to end South African control of the territory also known as Southwest Africa.

The official cautioned that the success of the talks would depend upon agreement by Angola, Namibia's pro-Soviet northern neighbor, to the withdrawal of the more than 15,000 Cuban troops in coordination with departure of South African troops from Namibia.

"Without that I cannot say these negotiations will succeed," he said.

The United States is heading a five-nation Western group seeking to negotiate a settlement between South Africa and the internal parties it supports in Namibia, on the one hand, and the South West Africa People's Organization (SWAPO) guerrilla group, backed by black African nations.

The official noted the "great diplomatic activity" among the parties in Africa and Europe in the last month, including a meeting this week in Luanda, Angola, between President Eduardo dos Santos and U.S. diplomatic troubleshooter Gen. Vernon Walters.

For months the talks have bogged down over SWAPO's refusal to accept a Western proposal for complex elections involving both constituency and proportional voting in the sparsely populated territory which is twice the size of California.

It is that issue which is now being "finessed" to move on to the second phase. The official said there was a consensus to "put it aside for resolution later."