Prime Minister Ian Smith blamed Britain and the United States yesterday for the failure of the March internal settlement to take hold and conceded for the first time that black-majority rule probably will have to be postponed "a few months" beyond the original Dec. 31 target date.

A a press conference, he also announced that the biracial transitional government was banning the black nationalist parties of Joshua Nkomo and Robert Mugabe operating inside the country as part of its new campaign to "liquidate" the internal opposition.

He attacked Nkomo as "a monster of no mean proportions" and seemed to be shutting the door on any further negotiations with the co-leader of the externally based Patriotic Front guerrilla forces.

"Certainly as far as I'm concerned I haven't left any doors open," he said. "I would sincerely hope I don't have to have any truck with the gentleman concerned. He's put himself out of court," he added, referring to Nkomo's taking responsibility for the shooting down of an Air Rhodesia passenger plane 12 days ago.

Smith's acid comments still did not rule out together the possibility of future contacts with Nkomo, whom he met secretly in mid-August in Zambia. But his words, plus the government's new ban on Nkomo's party and arrest of at least 320 of its officials, seem certain to complicate any future British-American initiative.

As for the future of the faltering seven-month-old transitional government, Smith indicated he still intended to push ahead with the program of holding both a white referendum on a new constitution and elections for a black-majority government.

He said he had doubted from the beginning that the elections could be held by Dec. 31 adding: "If the thing spills over for a few months, as a practical person, I suppose we've got to accept it."

The white Rhodesisn leader made clear that his immediate objective now would be to persuade Britain and the United States to recognize the multiracial transitional government set up under the agreement reached here in March between him and three moderate black leaders.

"The greatest contributing factor to our failure has been that the American and British government, the leaders of the free world, have sided with out enemies, the Patriotic Front, as opposed to siding with us," he said.