The Soviet Union and Cuba appear to have blocked a bid by Angola to offer new proposals to the United States on the withdrawal of Cuban troops from the war-torn country in southern Africa, according to a senior administration official.

The official said a high-level Cuban delegation visited Luanda just before the visit there of Assistant Secretary of State Chester A. Crocker, who talked with Angolan officials about the Cuban troop withdrawal issue in the Angolan capital July 14-15.

"We sense the Cubans are telling the Angolans it's all or nothing," the official said. Angola proposed a partial pullout in its last formal proposal, made in late 1984.

The United States had had "extensive indications" from the Angolans that they were prepared to make "fresh proposals" on the Cuban withdrawal issue. But the official said Crocker found that Angolan negotiators had "virtually no instructions" and presented only a reformulated version of their 1984 position.

The official suggested that Moscow also may have played a role in sabotaging a resumption of the U.S.-Angolan contacts and negotiations that had been suspended for 15 months.

He said Soviet Deputy Foreign Minister Anatoly Adamishin had told Crocker during a July 2 meeting in London that Moscow believed the negotiations were "stalemated" and that it was "fruitless" to continue unless their "substance" changed, an apparent reference to the Cuban troop withdrawal issue.

Whether the Angolans had a last-minute change of heart about making a new proposal to the United States is questionable. Angolan President Jose Eduardo dos Santos told visiting American reporters June 30 -- two weeks before Crocker's trips to Luanda -- that "as long as {South African} apartheid continues to threaten Angola, Cuban troops will stay in this country." This is the position Cuban leader Fidel Castro has articulated for some time.