MOSCOW: The sight of the Bolshoi Ballet's artistic director is improving after an operation in Germany to save his vision following an acid attack, Russia's chief ophthalmologist said on Wednesday.

The Jan. 17 assault put the spotlight on the bitter rivalries at the Bolshoi, one of the world's leading theatres, and in the fiercely competitive world of ballet.

Sergei Filin, 42, underwent several operations in Moscow before flying to Germany last week for more treatment after sulphuric acid was thrown in his face by a masked attacker.

"His left eye no longer causes so much concern. The right eye is still in a rather complex condition. But in any case, everything seems positive and optimistic now," Interfax news agency quoted eye specialist Vladimir Neroyev as saying.

The agency said a Russian eye doctor had been present during the surgery in Germany but gave no further details.

The Bolshoi has said German doctors now treating Filin have requested that information on his health and medical procedures are kept to a minimum to protect his privacy.

Such a move might also, however, help reduce the intense media attention on a saga that has damaged the Bolshoi's image.

Neroyev said last month that he expected Filin's sight to recover enough for him to be able to work again after a long period of rehabilitation.

Police have not arrested anyone over the attack. Filin says he believes he knows who was behind it and has made clear that he thinks it was connected to his work, in which he has the power to make or break careers.

Filin has not revealed any names in public but police are treating some within the troupe as suspects.