Burmese and Karen troops exchanged fire in the eastern border town of Myawaddy on Monday morning and many shots crossed into the Thai border, forcing evacuations of people in Tak's Mae Sot district.

Tak govenror Samart Loyfa said he had instructed authorities to close down the road to Burmese border and evacuate Thai residents and traders from the area immediately.

Mr Samart said many shells landed on Thailand – Burma Friendship Bridge and outside Mae Sot's immigration checkpoint.

Some Thai people were injured from stray bullets. They were sent to Mae Sot Hospital for treatment, he said.

Fifteenth Infantry Regiment commander Supachoke Thawatpeerachai said troops had been reinforced along the Thai-Burmese border.

"The Thai army will retaliate with heavy weapons if the situation is out of control and affect Thai people," Col Supachoke said.

Third Army Region commander Wanthip Wongwai said the checkpoint at Mae Sot had already been closed for awhile and the clash in Myawaddy should not affect Thailand.

"Thai soldiers have been ordered not to fire back but to monitor the situation very closely," Lt Gen Wanthip said.

Gunshots were heard continually at the border area, reports said.