South Africa's new prime minister, Pieter W. Botha, yesterday rebuffed a United Nations decision to push for independence of Namibia (Southwest Africa) and declared his country would not be cowed by "those who think they can chase us around."

His comments at a military airport near here came a day after the U.N. Security Council voted 12-0 to send up to 7,500 troops to the diamond-and-uranium-rich former German territory to monitor independence elections that South Africa has unilaterally decided to hold Dec. 4.

Botha, 62, elected prime minister Thursday to replace the ailing John Vorster, warned the world not to underestimate South Africa.

Botha, who will continue to hold the defense minister's job, said "While we do not think we can fight the whole world, I want to give those who think they can chase us around his assurance: Don't underestimate our determination to stay in this country. Don't underestimate our will to safeguard South Africa and the integrity of its borders. Don't underestimate our will to keep this an orderly community of nations.

"As far as Southwest Africa is concerned, don't underestimate our determination to keep peace so that the territory can find its way to self-determination in an orderly way," he added.