3rd Army commander Lt Gen Preecha Chan-ocha said on Sunday the two army districts filed the complaint with police in the morning, but did not elaborate.
Lt Gen Preecha said the 3rd Army had followed reports on secessionism and ordered the Chiang Mai and Phayao army districts to file a police complaint against the group under Sections 113 and 114 of the Criminal Code.
Section 113 deals with actions deemed intended to abolish or change the Constitution, topple the legislative, executive and judicial powers under the constitution, or separate the land or take over the administration of it. A violator is liable to life imprisonment or death.
Section 114 is about accumulation of forces or arms or preparations for an act of rebellion. A violator is liable to three to 15 years imprisonment.
Lt Gen Preecha said as director of the 3rd Region of the Internal Security Operations Command (Isoc), he had called a meeting of the governors of the 17 northern provinces and asked them to monitor and investigate any action which can be construed as affecting national security.
The governors agreed to cooperate.
The 3rd Army was also closely watching the activities and movements of the Sor Por Por Lanna group and the Rak Chiang Mai group.
Investigation results would be reported to the army chief at a meeting of army units to be held Monday.
Lt Gen Preecha said some groups of people in the area under the jurisdiction of the 3rd Army might have caused a misunderstanding and confusion among local people, especially the Sor Por Por Lanna group, which might be construed as the establishment of a new state in violation of the Criminal Code and the Constitution.
Sources said the name "Sor Por Por Lanna" might have led to a misunderstanding. It actually stands for Samatcha Pokpong Prachathipatai of Lanna (council of Lanna people to protect democracy) or Sor Por Por Lanna. But the abbreviation Sor Por Por Lanna is simillar to Sor Por Por Laos or the People's Democratic Republic of Laos, which is a form of socialist state.