Rhodesian Prime Minister Ian Smith met for more than three hours yesterday with South Africa's Prime Minister John Vorster amid increasing speculations that Western nations may threaten South Africa with an oil embargo in an effort to force Smith to accept the Anglo-American plan for black-majority rule in Rhodesia.

Land-locked Rhodesia, which is under a U.N. trade embargo, receives most of its imported supplies through South Africa. Vorster is believed to be under strong pressure to help get a Rhodesian settlement.

South Africa's economics minister, Chris Heunis, also attended the talks in what observers interpreted as an indication that the possibility of international economic sanctions was discussed.

Following the meeting, the fifth between Smith and Vorster this year, South Africa's Foreign Minister R. F. (Pik) Botha said: "We reviewed the southern Africa situation up to the present time. We have nothing more to add."

Meanwhile, two rival Rhodesian black nationalist leaders passed through Johannesburg en route to a meeting with President Kamazu Banda of Malawi. The two - Bishop Abel Muzorewa and the Rev. Ndabaningi Sithole - appeared to be seeking black support in an effort to counter the backing two other nationalist leaders, Joshua Nkomo and Robert Mugabe, have received from Angola, Zambia, Tanzania, Botswana and Mozambique.