Ms Yingluck was responding to reports that army chief Prayuth Chan-ocha has instructed the Third Army to pursue legal action against a red-shirt splinter group running activities which could be construed as seeking to separate the country and form a new state.
The prime minister said the government does not support separatism, adding that the authorities would investigate whether more than one group was involved in the alleged separatist movement.
Ms Yingluck said she does not believe that such a movement exists.
"I want to see Thailand as being one and inseparable, including the three southern border provinces where problems are being solved through negotiations," Ms Yingluck said.
Asked whether red-shirts groups would be warned about attempting to partition the country, the prime minister said all sides would be told not to break the law. Judicial process and proper legal procedure will be used to deal with detractors, she said.
The premier said she was happy for the army to take legal action against any group promoting separatism, but said it must treat all groups that seek to divide the country in the same way, so that no group is discriminated against.
Asked whether army chief Prayuth Chan-ocha would report to her on the progress of the case against the red-shirts, Ms Yingluck, who is also defence minister, said Gen Prayuth could do so in his capacity as deputy director of the Internal Security Operations Command.
Asked whether the emergency decree would be lifted now that the protest situation in Bangkok had improved, Ms Yingluck said the decision rested with the Centre for Maintaining Peace and Order.