A spokesperson for the Myanmar junta did not respond to calls for comment.
The KNU said last week that its troops had attacked a junta camp near Myawaddy, forcing about 600 security personnel and their families to surrender.
Border crossings in the area were open for civilians who have been coming into Thailand from Myanmar in large numbers, said Police Colonel Borwornphop Soontornlekha, Superintendent of Immigration in the Thai province of Tak, where Mae Sot is located.
“Usually there are about 2,000 people who cross into Mae Sot from Myawaddy each day but the last three days the number was almost 4,000 per day,” Borwornphop told Reuters.
Thailand’s military has stepped up security on its side of the border, deploying army vehicles equipped with roof-mounted machine guns.
At least 2,000 people have been displaced within Myanmar by the latest round of fighting between the rebels and the military, according to civil society group Karen Peace Support Network.
Myanmar’s military, which took power in a 2021 coup after deposing an elected civilian government, has faced a series of setbacks against a loose alliance of ethnic rebel groups and a civilian militia movement.