Soldiers in Tak province have been put on high alert after heavy fighting between Myanmar government soldiers and Karen rebels temporarily closed the Mae Sot border pass.
Lt Gen Preecha Chan-o-cha, commander of the 3rd Army, said the clashes erupted four to five kilometres from the Thai border.
Myanmar soldiers asked their Thai counterparts to close the Mae Sot pass, but it was reopened late afternoon yesterday after fighting died down.
Despite the lull in violence, Prime Minister and army chief Prayut Chan-o-cha has placed Thai soldiers on high alert and ordered reinforcements along the border in case the conflict reignites.
Mae Sot district chief Preecha Jaipetch said five were reportedly killed in the clashes, while more than 10 were injured. The dead and injured were said to be from both sides.
Heavy fighting started at 10.30am, with rocket-propelled grenades and mortars fired in the centre of Myawaddy town, security sources said.
Exchanges of gunfire then continued until after noon. The noise of shooting could be heard clearly over the Thai border.
Sources said hostilities between Myanmar government soldiers and Karen rebels near the Mae Sot border have been escalating since August, when the Democratic Karen Benevolent Army allegedly started blocking imports from Thailand and demanding "tax" payments on the goods.
One Karen soldier was killed in the area last week and there have been several related bombings in Myawaddy.
The violence prompted the Myanmar military to prohibit all soldiers from armed Karen factions from entering urban areas in uniform or with weapons. Karen troops responded by banning Myanmar soldiers from carrying weapons in Karen-dominated areas. The latest clashes were caused by breaches of the "bans".