A senior Cuban official has said Cuba now has 40,000 troops stationed in Angola, a figure higher than any U.S. intelligence estimate to date and one that seems to confirm reports of a new Cuban military buildup in that war-devastated southern African nation.

The figure is at least 3,000 more than the highest used by the Central Intelligence Agency and Defense Intelligence Agency and 5,000 more than a Cuban air force general defector said were present in Angola early last summer.

The Cuban buildup is apparently in response to recent Angolan government setbacks in its 12-year-old war with U.S.-armed rebels and increased South African military involvement in southern Angola. The two developments have led to a decision by Cuban leader Fidel Castro to commit Cuban troops for the first time to an active role in Angola's effort to repel South African military incursions, according to Angolan sources.

The expanded Cuban role comes as the Reagan administration is renewing its diplomatic efforts to persuade Angola to send the ever-increasing Cuban contingent home and South Africa to grant independence to neighboring Namibia.

Assistant Secretary of State Chester A. Crocker is scheduled to leave this month for a new round of negotiations with Angolan authorities in Luanda on these two issues, which have been linked by the Reagan administration. It has insisted that the Cuban troops must leave before the United States will extend diplomatic recognition to the Marxist Angolan government.

The figure of 40,000 Cuban troops appeared this past week in the Cuban magazine Bohemia, which reprinted an interview by a Swedish correspondent with Jorge Risquet, a member of the Cuban Communist Party's ruling Politburo.

Risquet dismissed an allegation by the former Cuban Air Force general, Rafael del Pino Diaz, who defected last June and has estimated Cuban war casualties in Angola at 10,000 dead and wounded since the Cuban troops first arrived there in March 1975.

Risquet said 1,000 Cubans have died but did not give a figure for the number of wounded. He also said the majority of deaths were the result of accidents and disease rather than combat.

Del Pino, in an interview with The Washington Post Dec. 16, put the number of Cuban troops in Angola as of the time of his defection at 35,000. He also said he knew of only two incidents in which South African and Cuban forces had clashed in southern Angola over the past decade.

There was "a silent agreement" between Cuba and South Africa that the Cubans would not go south of, or the South Africans north of, the 16th parallel, and an "order from Havana not to provoke any clashes with South Africans," del Pino said.

The two incidents, he said, had occurred in 1980 and 1982 and involved two Cuban-piloted planes, apparently on support missions for the Angolan army, that were so badly shot up by the South Africans they could not be repaired and were scrapped.

Del Pino said he was the first Cuban officer assigned to Angola in March 1975, even before the departure of the ex-colonial power there, Portugal. He said he remained involved in the Cuban expeditionary force in Angola for years.

Reports of new Cuban troops arriving in Luanda surfaced in the Mozambican press early last month and were disclosed by Angolan authorities, apparently in a bid to force South Africa to withdraw an Angolan-estimated 3,500 troops it had sent in November into southern Angola.

A New York Times report Dec. 15 from Luanda, quoting Angolan authorities, said Cuban troops had begun patrolling in southern Angola with orders to engage South African troops in combat. The report said Angolan President Jose Eduardo dos Santos had met with Castro in Moscow Nov. 7 and that the two leaders had agreed to replace freshly recruited Cuban troops assigned to Cuba's Fifth Division with battle-experienced ones.

Risquet's figure of 40,000, however, suggests that Cuba has sent additional troops rather than just replacements.