Army chief Apirat Kongsompong has ordered that security be boosted along the Thai-Malaysian border after 672 Thais sneaked into Narathiwat province without undergoing health screening.
The elevated security will respond to national security concerns, said the army chief.
"We need to identify them, find out where they live and check if they have criminal records," Gen Apirat said on Tuesday as he discussed the situation with military officers stationed along the Thai-Malaysian border in Narathiwat's Sungai Kolok district.
The army will ramp up its monitoring of the border in the restive South as 8,000 Thai labourers in Malaysia are expected to return home.
Over the past week, there have been growing reports of illegal border crossings as hundreds of Thai workers have opted to pay fines rather than secure the necessary paperwork to return home.
The army chief said he was also concerned that separatist groups might exploit the situation to enter the country.
Gen Apirat has ordered background checks for all those who sneaked across the porous border in Narathiwat district this past weekend.
He said the illegal returnees would be forced to undergo health checks and mandatory quarantine.
The army chief on Tuesday visited military officers stationed along the border in the area, which serves as a hot spot for illegal entry.
Along the 45km border, there are 96 natural border passes.
The prime spot for illegal entry is located along the Sungai Kolok River in a place where the river is shallow, enabling smugglers to swim across or even wade across at night.
The army chief ordered more lights to be installed at suspected crossing points and will deploy a team of 37 soldiers to patrol them.
Meanwhile, 100 returnees were registered on Tuesday at Sadao border checkpoint in Songkhla.
There were no reports of infections among them, but one immigration officer tested positive for the coronavirus.
The latest case brought the total number of patients in Songkhla to 38, after the province had gone 15 days without a new infection.
In Yala province, 81 Thais have returned since Saturday, all of whom were sent to a state facility to undergo quarantine.
In addition, Thais are continuing to return from other countries, including 100 from Japan and 120 from Taiwan on Tuesday.
