HAVANA, May 21 (Xinhua) -- The three survivors of last week's tragic plane crash in Cuba are still in extremely critical condition, the director of Havana's Calixto Garcia Hospital, Carlos Alberto Martinez, said on Monday.

The remaining 110 people on board were killed after the aircraft plummeted to the ground and burst into flames just a kilometer from Havana's Jose Marti International Airport, where it had taken off.

"The medical team is performing surgeries and carrying out the treatments responsibly, in keeping with the situation presented by the patients," Martinez told reporters at a press conference.

However, the extent of the injuries made the prognosis bleak, especially for Grettel Landrovell, 23, who had to be operated on again for an injury to the brain. An incision was made to decompress the area and improve circulation.

The head of the medical team said the mere fact the survivors were still alive 72 hours after the incident was a "step forward" in the complicated process.

"We have been able to adopt the necessary measures to sequentially correct remaining alterations and those that may appear later, after eliminating the problems from the initial evaluation," he said.

Still, the survivors run a high risk of future complications, he added.

Mailen Diaz, 19, underwent surgery on the cervical spine and has shown improvements in neurological responses, while Emiley Sanchez, 39, was being treated for sustaining burns over 41 percent of her body.

Officials don't yet know what caused the demise of the plane, which Cubana airlines rented from Mexican charter airline Damojh for a domestic route.