Angola and Cuba have jointly agreed for the first time to a total withdrawal of the 40,000 Cuban troops in Angola as part of a still-unresolved overall southern Africa peace settlement, the State Department said yesterday.

The agreement, which came during talks with Assistant Secretary of State Chester A. Crocker late last week in Luanda, the Angolan capital, is regarded within the administration as a potential major breakthrough in long-stalled U.S.-led negotiations with Angola and South Africa.

The talks have been aimed at arranging for the simultaneous withdrawal of Cuban troops and South African forces from Angola, as well as elections for independence in neighboring Namibia, which South Africa administers under a disputed League of Nations mandate.

State Department spokesman Charles E. Redman said there is "no doubt" that the Angolan-Cuban troop withdrawal offer is "an important step" and "an important missing element" in the negotiations to date.

Cuban Politburo member Jorge Risquet participated in the talks, another unusual aspect of Crocker's latest round of negotiations with Angolan authorities. The United States has no diplomatic relations with Angola and only limited relations with Cuba.

"The Angolan delegation for the first time affirmed its acceptance of the necessity of the withdrawal of all Cuban troops from Angola in the context of a settlement," Redman said. "Cuban officials concurred in this decision."

The Angolan government has said it submitted unspecified "new proposals" to Crocker. A source close to the Angolan authorities said he understood that the new Angolan-Cuban commitment to a complete Cuban troop withdrawal is among them.

Redman said "several Cuban representatives" led by Risquet had joined the Angolan delegation "briefly for exchanges relating to Cuban troop withdrawal." The Angolans had asked that the Cubans be included to discuss the withdrawal issue, and the United States agreed on "the understanding" that this would lead to progress toward "concrete proposals" on this issue, he said.

The reported Angolan-Cuban offer comes against a backdrop of recent heavy fighting in southern Angola between the Soviet- and Cuban-backed Angolan army and U.S.-armed and South Africa-supported guerrillas led by Jonas Savimbi's National Union for the Total Independence of Angola (UNITA).

In December, Cuba sent additional and better-trained troops to help the Angolan army, and there have been reports of Cuban pilots flying combat missions against UNITA rebels and South African units operating in southern Angola.

The reported Cuban withdrawal offer as part of an overall settlement appears to signal a shift in Cuban policy. Cuban leader Fidel Castro had earlier threatened to keep his troops in Angola until there is an end to apartheid in South Africa.

Redman said the United States awaits an Angolan initiative to close the gap on a troop withdrawal schedule between an earlier Angolan offer and a compromise U.S. proposal.

"When they do, we'll be ready to meet with them again to proceed with the achievement of a regional peace settlement," he said.

A U.S. official said the United States would wait until it receives a specific Angolan schedule before going to South Africa with it. "It's squarely in their court to come up with a timetable," he said.

Angola has never offered any timetable for a complete Cuban troop withdrawal. Its proposals for a partial pullout have been unacceptable to the United States and South Africa, and talks among the parties have proceeded intermittently in recent years, with no agreement on the pace or timing of a pullback by outside military forces.

Still unclear is the Angolan position on a cutoff of U.S. military and other assistance for UNITA and whether a total Cuban troop withdrawal is dependent not only on a South African pullout from Namibia but also an end to American support for Savimbi.

In the past, Angolan officials have linked a cessation of U.S. aid to an overall settlement. But the administration has carefully sought to separate the negotiations over Namibia and a Cuban troop withdrawal from the question of UNITA's future in Angola, pressing for national reconciliation as a separate issue.