Maoist rebels on Sunday night mounted an attack on a Central Industrial Security Force (CISF) camp near a Nalco-run mine in Orissa's Koraput district, said the police. "The rebels have surrounded the CISF camp at Panchpatmali Hill at about 9.15 pm. An exchange of fire between CISF personnel and the rebels is on," said district SP Deepak Kumar."We are not able to communicate with the camp. We are moving additional police forces to the region," he said, adding that he was not in a position to disclose more details.