Envoys of the United States and four other Western countries will strongly press South African Prime Minister John Vorster in talks Thursday to hasten the independence of Namibia (Southwest Africa) by allowing free elections, diplomatic sources here said today.

A firmly worded message to be given Vorster in Cape Town will also call on the South Africa told reporters.

There were conflicting reports here and in South Africa today about how the South African government is likely to respond to the unusual diplomatic move by the ambassadors of the United States, Britain, Canada, France and West Germany.

According to Western diplomats here, South Africa has hinted that it will agree to a demand that it enter negotiations with the five countries to set up a system of free elections in Namibia, which it administers in opposition to U.N. directives.

In South Africa, government sources told reporters that the demands that they understood would be made in the Western countries' note were not acceptable.

South African political sources said, according to United Press International, that the five ambassadors should have waited until after Vorster and Rhodesian Prime Minister Ian Smith meet next week with British Foreign Secretary David Owen to discuss racial problems.

The Namibia initiative is linked to negotiations among Security Council members, which have been going on behind the scenes for more than three weeks, on a "declaration of principles" to be issued by the Council on all the problems of southern Africa, including Namibia.

The original hope of U.S. Ambassador Andrew Young, who has sponsored both the declaration and the approach to South Africa, was that the declaration would be complete by the time of Thursday's meeting, and could be presented to Vorster along with the request for a Western role in the solution to the Namibia problem.

But negotiations on the declaration between the West and African nations at the United Nation have reached an impasse. Western diplomats now hope that the approach to Vorster will demonstrate their commitment to activism on southern Africa, and encourage the Africans to reach a compromise on the declaration.

Western officials said they hope to demonstrate that the West canf produce movement in southern Africa, to counter the diplomatic and military initiative that have been launched in the region by the Soviet Union.

Sources said the decision was made to concentrate on Namibia because of South Africa's intention to establish an interim government for that territory in June or July that would violate U.N. guidelines on Free elections and would omit the Southwest Africa People's Organization (SWAPO), the leading liberation movement in the territory, from the process of self-determination.

A Western diplomat who has been involved in the negotiations said, "We don't expect Vorster to say no outright. We are confident he'll accept talks with the five of us, at least to have a look at the Namibia situation."

If south Africa does accept the Western proposal, "idea is for this contact group to start in South Africa before the end of the month, at a fairly senior level," he said.

The reaction of African dimpomats to the impending Western move was cautious. Many expressed doubt that South Africa would cooperate in anything more than a delaying action.