A senior Rhodesian government official will meet today with top U.S. and British representatives in Pretoria to hear proposals for a new conference bringing together all the parties in the 12-year-old Rhodesian constitutional dispute.

The talks, to be held in the British Embassy, will be an attempt to persuade the white-dominated Rhodesian government of Prime Minister Ian Smith to meet with the two black nationalist leaders, Joshua Nkomo and Robert Mugabe, who did not accept the settlement reached earlier this month between Smith and three black leaders living inside Rhodesia.

It will be the first time that a U.S. official has met with the Rhodesians since their internal settlement was achieved.

That settlement, reached after three months of hard bargaining, paved the way for a black-run government at the end of this year in Rhodesia, but with some limitations. Nkomo's Zambia-based Zimbabwe African People's Union and Mugabe's Zimbabwe African National Union, which together form the Patriotic Front, refused to accept the Salisbury settlement because they said it perpetuated rule of the white minority in Rhodesia.

Both Mugabe, whose guerrillas are based in Mozambique, and Nkomo, with an estimated 5,000 Cuban-trained troops in Nambia, said they will intensify their war against any government set up by Smith and the internal black leaders.

The Security Council also rejected the Salisbury agreement earlier this week.

The United States is to be represented by an officer of its embassy in South Africa while Britain is sending Deputy undersecretary John Graham, who is an expert on the Rhodesia question. The Briton and American "will explain their thinking in connection with a new, broader Rhodesian conference," a British spokesman said.

At the same time, the meeting will "afford an opportunity to explain to British and U.S. representatives those parts of the Salisbury agreement which have been criticized by British and American spokesmen," the Rhodesian government said in Salisbury. Jack Gaylard, secretary to the Rhodesian ZCabinet, is to head the Smith government's representation.