Gunmen armed with assault rifles opened fire "indiscriminately" at the mosque in the Narathiwat province where around 50 people were attending evening prayers, the paper quoted the army and police as saying.
"They opened fire indiscriminately at about 50 worshippers inside the mosque,'' a police official was quoted as saying.
He added the local imam was also killed and that that up to five people had been involved in the attack.
Army spokesman Col. Parinya Chaidilok, however, said there were two attackers, one of whom entered through the front door of the building while the other came in by a side door before opening fire.
He confirmed that 10 people were killed instantly and another died on the way to the hospital, while the 12 wounded were all in critical condition.
Authorities have previously blamed most of the violence in the country's south on shadowy, separatist Muslim militants.