U.N. Secretary General Kurt Waldheim asked the Security Council yesterday to send a United Nations force of 7,500 troops and 1,200 civilian officials to Namibia (Southwest Africa) to preside over that territory's transition to independence from South Africa.

Waldheim, in a report to the Security Council, said the U.N. task force would probably have to remain in Namibia for 12 months. He estimated the cost of the operation at $300 million.

The size of the operation - which would be the largest since the United Nations sent troops to restore order to the former Belgian Congo, now Zaire, in the early 1960s - took many U.N. delegates by surprise, Waldheim had said earlier this summer he thought 5,000 troops would be sufficient.

South African Foreign Minister Roelof "Pik" Botha, whose country had insisted that the U.N. force should be kept below 5,000 men, declared last night that Waldheim's proposal created "severe problems" for his country.

Both Botha and Sam Nujoma, whose South West Africa People's Organization (SWAPO) guerrillas have been attempting from South Africa, were reported en route to the United Nations last night.

South Africa has ruled the territory since 1920 under a League of Nations mandate that the United Nations revoked in 1966. South Africa and SWAPO both agreed last month to a Western plan under which Namibia would finally gain full independence.

While the Security Council may take up Walheim's proposal for implementing the plan Friday, diplomatic sources said it probably would not come to a vote until at least Tuesday.

If the black African nations continue to support the Western plan for a U.N. force to preside over the transition to Namibian independence, diplomatic sources said it seemed unlikely that either the Soviet union or China would veto the Waldheim proposal.

While Security Council approval would put the plan in motion, paying for it may be more complicated. The General Assembly will have to give its approval to the overall financing, and U.S. sources said they expected that the Soviet Union, and perhaps China, would refuse to pay their share of the cost of the peacekeeping operation.

Waldheim's proposal calls for a start of deployment of U.N. troops in the vast, mineral-rich territory three weeks after the Security Council approves creation of the force which would be called the U.N. Transition Group (UNTAG).

While neither American nor Soviet troops customarily take part in U.N. peacekeeping operation, U.S. diplomatic sources said last night that the United States would be willing to fly the U.N. forces into Namibia.

Waldheim said the force should have "a strength of the order of seven infantry battalions, totalling approximately 5,000, plus 200 monitors, and in addition, command, communications, engineer, logistic and air support elements totalling 2,300."

Under the plan, the U.N. force - which will reach full strength within nine weeks after its begins moving into Namibia - will monitor a ceasefire between South African troops and SWAPO guerrillas, and the withdrawal, containment or demobilization of their forces.

Waldheim said in his report to the Security Council that the U.N. troops "will not use force except in self-defense."

"Self-defense," he added, "will include resistance to attempts to prevent it from discharging its duties under the mandate of the Security Council."

The U.N. force would also be charged with conducting "free and fair elections" in the territory, Waldheim proposed yesterday that elections for a constituent assembly he held approximately seven months from the day the Security Council approves his report.

This point, which would appear to mean April elections is also a problem for South Africa, which earlier this year committed itself to independence for Namibia by Dec. 31.

Botha, speaking to reporters at Johannesburg's Jan Smuts Airport last night, said his government's position was that an election for a constituent assembly must be held this year so it can"decide for itself whether it wants to retain" the independence date.

Under Waldheim's proposal, the U.N. troops would remain in Namibia for approximately five months after election of a constitutent assembly to ensure that a constitution is adopted before Namibia declares itself independent.

This would mean that Namibia would actually gain independence at about this time next year.

Waldheim's report yesterday was based on a 17-day fact-finding mission to the territory this month by Martti Ahtisaari of Finland, the U.N. commissioner for Namibia.

One U.S. source said the troop figure recommended by Ahtissari was apparently higher than expected because "nobody had thought about military support personnel."

Another U.S. source also noted that a force of 7,500 troops was probably the minimum that could be expected to maintain order in such a large territory since South Africa has kept 23,000 to 28,000 troops in Namibia to perform the same task.

The civilians who will be sent to Namibia under the Waldheim proposal will serve primarily as observers during the election campaign and the balloting. Many of the civilian officials are expected to be posted to the territory for considerably less than a full year.

While the United Nations has dispatched peacekeeping forces to a number of other hotspots, including Cyprus and the Middle East, over the past two decades, the Namibian operation will be the largest since the U.N. involvement in the Congo.

The Congo operation, which lasted from 1960 to 1964, cost the United Nations $425 million. At the peak of the operation, the United Nations had 23,400 troops in the Congo.