A Rhodesian government spokesman said yesterday that an American nun arrested last week will remain in custody until after a preliminary hearing, for which no date has been announced.

In Washington, a State Department spokesman said the U.S. ambassador in South African has been asked to look into the case of the Maryknoll nun, Sister Janice McLaughlin, 35, of Pittsburgh. The United States and Rhodesian do not have diplomatic relations.

Three other members of a Roman Catholic human-rights commission arrested with her last week appeared in court Thursday and released on $1,700 bail each, but the Rhodesian official said McLaughlin's case is "more complex."

A police spokesman said, "She will be appearing in court in due course."

The four Catholics are believed to have investigated allegations that Rhodesian soldiers tortured black prisoners. A report by the Catholic Institute for International Relations to be published this week in London reportedly includes their findings.

Prosecutor Amos Shirwa said the four were arrested after police classified documents during a search of their office and quarters. McLaughlin came to Salisbury in Jine as information officer of the human-rights commission. She had previously been stationed in Kenya.

A Church spokesman said her temporary employment permit expires today and it is expected that she will be expelled. The others arrested include two Rhodesians and a West German.

There were these other southern African development:

Black nationalist leader Ndabaningi Sithole appeared to Rhodesia's whites tos support the Anglo-American plan to end the guerrilla war and bring about black-majority rule.

"We need the white man's skill," the American-educated minister told plantatation owners at a segregated country in Salisbury.

Zambian President Kenneth Kaunda called on the West and the world's major oil-producers to impose a ban on fuel shipments to Rhodesia.

African sources in Paris said that South African Foreign Minister Roelof (Pik) Botha conferred with ivory Coast President Felix Houphouet-Boigny in Geneva Sunday on the disputed future of Namibia (Southwest Africa). Israeli radio said Botha met with Prime Minister Menahem Begin during the South African's visit to Jerusalem last week.