The Pheu Thai Party and caretaker government downplayed the "separatist" threat, after the issue was seized upon by the army and protesters.
Yingluck Shinawatra arrives at the Centre for Maintaining Peace and Order on Monday, where she made a statement to condemn any attempt to end national unity. (Photo by Chanat Katanyu)
Pheu Thai issued a statement confirming the fact that it totally opposes any attempt to split the country and strongly supports national unity.
Party spokesman Prompong Nopparit said the party urged the army to take legal action against those floating the idea of partitioning Thailand.
Caretaker Prime Minister Yingluck Shinawatra took the same tone, saying the government fully supports national unity and insisting that news reports about possible separatist action in the North need further investigation.
Ms Yingluck was responding to reports that army chief Prayuth Chan-ocha has instructed the Third Army, which is responsible for the North of the country, to pursue legal action against red-shirts running activities which could be construed as seeking to separate the kingdom and form a new state.
The caretaker prime minister said the government condemns separatism, adding that the authorities would investigate whether more than one group was involved in the alleged separatist movement.
The issue first surfaced on Jan 28, when a vinyl banner bearing a "split Thailand" message was hung on a pedestrian flyover in Muang district, Phayao province.
It read: "There is no justice in this country. We want to separate to form Lanna country."
The same type of banners have since been found displayed in other provinces, including Chiang Mai.
Phayao and Chiang Mai are political strongholds of the Pheu Thai and the red-shirt United Front for Democracy against Dictatorship (UDD) group.
On Gen Prayuth's order, the army on Monday filed a police complaint against Petcharawat Wattanapongsirikul, a core member of the Rak Chiang Mai 51 group, alleging separatism and treason under Sections 113 and 114 of the Criminal Code.
The complaint was filed at Mae Ping police station by Col Pokha Jokloy, chief of the Information Division of the 33rd Army District.
Mr Phetcharawat was accused of separatist activities in the name of the Sor Por Por Lanna group, which put up a banner calling for northern independence on a pedestrian bridge over Don Chan intersection in Chiang Mai.
Col Pokha's complaint was accompanied by photo evidence of the group's activities, including the cloth banner.
The same complaint was filed with Muang Chiang Mai district police.
Caretaker Deputy Prime Minister and Foreign Minister Surapong Tovichakchaikul came out in defence of the Chiang Mai red-shirts, arguing that they were not promoting separatism.
Mr Surapong, chief adviser to the Centre for Maintaining Peace and Order (CMPO), called on Gen Prayuth to be politically neutral. He added that people felt uneasy about the country's justice and political systems, which he claimed operate under "double-standards".
"I can give you an assurance that the northern people are not instigating separatism or division in society.
"They are only expressing their feelings about the justice system and its double-standards, particularly the Civil Court's ban on the CMPO's orders issued under the emergency decree," he said.
Unlike People's Democratic Reform Committee (PDRC) leader Suthep Thaugsuban, who had instigated unrest in the country, the Chiang Mai and Phayao red-shirts had fought for democracy, he said.