Top-ranking U.S. and South African diplomats will meet this week in the Cape Verde Islands to press what is shaping up as another critical phase in the Reagan administration's southern African diplomacy, according to well-informed diplomatic sources.

Assistant Secretary of State Chester Crocker will meet with South African Foreign Minister R.F. (Pik) Botha to explore a series of proposals and suggestions made to a U.S. special envoy by Angolan officials during recent talks in Luanda, the sources said.

"It is clear the door is open for a settlement. Both sides have made decisions in principle" to try to reach one, a senior official said. "There is a mood on both sides to get together."

The questions of independence for South African-controlled Namibia, the role of Namibian guerrillas operating from Angola, as well as Cuban troops in Angola and South African support for the Angolan guerrilla force of Jonas Savimbi, have become intertwined in a long-running set of diplomatic contacts that has been the focus of the four years of the Reagan administration's African diplomacy.

Suggestions of progress on the broader issues came to a halt last December, when South African troops thrust about 120 miles into southern Angola in bitter fighting that lasted about two months.

The fighting, however, led to direct contacts between Angolan and South African officials in Lusaka, Zambia, in February that resulted in a five-stage withdrawal agreement. South African troops did pull back to a point 25 miles north of the Namibian frontier, but they have been stalled there. The South Africans continue to operate joint monitoring patrols with Angolan troops.

A key official in contact with both sides indicated yesterday that the current round of diplomatic contacts is shifting the focus back to the larger diplomatic issues.

He pointed to Angolan leader Jose Eduardo dos Santos having signaled this approach publicly in an interview with The Washington Post 2 1/2 weeks ago, his first with an American newspaper in five years. Dos Santos said, "We have made many proposals directly and indirectly through the United States of America to break this impasse."

"He said what his diplomacy is," said the official, who has been intimately involved with the diplomatic process. "A decision has been reached to solve this problem."

The official said he did not expect this week's talks with the South Africans to be expanded immediately to include top-level Angolan officials, although Cape Verde has been the common meeting ground for the three parties.

Subjects likely to be discussed include Angolan security, the future of the estimated 25,000 Cuban troops in Angola and a date and procedure for implementing U.N. Resolution 435 on Namibian independence, the official said, cautioning that "we are not at a point of reaching a negotiated settlement."