A Congo Airlines executive said the victims were civilians being evacuated from Kindu, about 620 miles east of Kinshasa. Neither claims could be confirmed independently.
Stavros Papaioannou, the president of the airline's management committee speaking in Kinshasa, said a missile hit the rear engine of the Boeing 727 jet after the aircraft took off from Kindu airport.
A rebel military commander, who requested anonymity, said the plane was shot down while approaching Kindu air base, where the rebels had deployed at least two surface-to-air missiles and more than 3,500 troops in preparation for a final assault on the government-held town.
The pilot struggled to bring the plane around for an emergency landing before sending a final notice that the aircraft would not make it back to the airport, the airline executive told The Associated Press.
In August, rebels took over several civilian CAL aircraft in eastern Congo after they launched their rebellion against President Laurent Kabila. The planes were used to shuttle soldiers to western Congo near the capital.
Arthur Mulunda, a rebel brigade commander, said his men had deployed two SAM-7 missiles to prevent government aircraft from flying in reinforcements and supplies.
Rebel aircraft from the eastern stronghold in Goma made at least eight flights to Kalima's grass air strip Saturday carrying ammunition and troops. All supplies and troops are trucked to the frontline at Kindu.
Mulunda said rebels controlled three-quarters of the perimeter around Kindu. He said a corridor leading southwest out of the town had been left open for the civilian population to flee and escape possible casualties.
After six days of battling for Kindu, rebel fighters ethnic Tutsis and disenchanted soldiers in Congolese army backed up by neighboring Rwanda had made little progress and failed to capture the town outright.
The rebels see Kindu as a major prize in their two-month revolt against President Laurent Kabila, whom they accuse of mismanagement, corruption and warmongering among Congo's 400 tribes.