Defectors from the Angolan rebel group UNITA alleged in a videotape shown to the U.N. Security Council today that UNITA's leader, Jonas Savimbi, ordered the downing of two U.N. airplanes a year ago, killing 23 passengers and crew members.

The charges were leveled in the unusual videotape by Gen. Jacinto Bandua and Lt. Col. Jose Antonio Gil, former UNITA commanders who are now at war with Savimbi and may have a stake in defaming him. Diplomats familiar with a U.N. probe into the crashes, which left one American dead, said there are discrepancies between the U.N. findings and the defectors' testimony.

"Savimbi issued specific, categoric orders to shoot down any aircraft of the United Nations," said Jacinto, dressed in army fatigues and sitting in front of a U.N. flag. "And it was he who, after the shooting, gave orders to cover it up so as not to find any trace."

Ambassador Robert Fowler of Canada, who questioned the officers in Angola last week, said there was no indication that their testimony was coerced or coached. Fowler said the two men, who had been UNITA's chief logistics officer and top air defense official in the region where the planes were shot down, were in a position to know what happened.

Gil said a rebel named Gregorio downed both U.N. aircraft--on Dec. 26, 1998, and Jan. 2, 1999--with a shoulder-held rocket launcher. Savimbi then ordered his men to destroy the planes' black boxes, burn any human remains and bury the evidence, according to the two officers. Gregorio later received a promotion, they said.

The officers said Savimbi was angry at the United Nations, believing that the world body was siding with Angola's government against his rebel army. The rebels have waged war against the government in Luanda for 25 years, since Angola's independence from Portugal.

Fowler said he has given copies of the videotape to the U.N.'s top security official, Benon Sevan of Cyprus.

Last January, Sevan assigned a team to film the debris at the crash site, collect evidence and deliver its findings to the International Civil Aviation Organization. While the U.N. investigators suspected UNITA as the culprit, they concluded that the first plane was flying beyond the range of a shoulder-launched missile and that the second aircraft was brought down by antiaircraft guns, diplomats said.

The new allegations came out of an investigation by a U.N. sanctions committee, headed by Fowler, into UNITA's military buildup in recent years. Fowler accused Savimbi of taking advantage of a U.N.-brokered cease-fire in the mid-1990s to prepare for war, which broke out again in December 1998.

In March, Fowler intends to publish a report identifying individuals and countries that have helped Savimbi's cause. He said several UNITA defectors, including one of Savimbi's children, Aruaja Sakaita, have provided information on the role of international arms dealers in supplying tanks, weapons and fuel to the rebels in exchange for diamonds.