U.S. mediators are working on a compromise plan for 50,000 Cuban troops to withdraw from Angola within 18 months, sources close to negotiations between South Africa, Angola and Cuba said.

But the plan, offered during talks scheduled to conclude Friday, is likely to be unacceptable to the Angolans and Cubans who were insisting on a minimum of two years, the sources said. They spoke Thursday on condition of anonymity.South Africa has promised to pull its troops out of Angola by Sept. 1 and grant independence to Angola's southern neighbor, South-West Africa, within a year, as long as the Cubans withdraw.

The Marxist Angolan government publicly has offered to send the Cubans home in four years.

Meanwhile, Jane's Defense weekly quoted Aubin Heyndrickx, head of toxicology at the University of Ghent in Belgium and a U.N. expert on biological and chemical warfare, as saying evidence based on environmental samples shows Cuba is using a lethal mixture of mustard gas and nerve gases to flush out South African-backed Angolan guerrillas from their rural strongholds.