The army has deployed rangers along the Thai border in Mae Sot district of Tak province following the escalation of fighting inside Myanmar, an army source said on Saturday.
The rangers are closely monitoring conditions amid tightened security as a pre-emptive measure due to the clashes between Myanmar government soldiers and the Democratic Karen Buddhist Army, the source said.
The fresh fighting began on Wednesday on Asian Highway 1 between Myawaddy town and Kawkareik in Karen state. The Myanmar Times reported on Friday that the clashes centred on a dispute over illegal tax collection on the road.
The Thai source said some sections of the road remained closed on Saturday but could not confirm the cause of the armed clashes.
The road enters Thailand at Mae Sot as part of the East-West Economic Corridor stretching from Myanmar to Vietnam through Thailand and Laos.
Thai security authorities have anticipated a new round of fighting as Myanmar troops and the Karen rebels had been building up personnel and weapons in the area about 50 kilometres inside the country.