Thai armoured cars patrolled the town of Mae Sot as the deep boom of artillery thundered across the border in Myanmar, where the junta and an ethnic armed group fought for a second day near a vital trade hub.

Hundreds queued to enter Thailand at the immigration checkpoint in Mae Sot, as many fled the newest round of fighting to test the junta's hold on power.

Thai soldiers took up positions underneath the bridge linking the town with Myanmar trade hub Myawaddy.

Above the soldiers, hundreds walked across the friendship bridge and into the safety of the kingdom.

Immigration said the number of people passing Thai immigration from Myanmar had increased to around 4,000 per day in recent days.

He said authorities were reinforcing the number of immigration officials to address the possibility that arrivals would rise further in the coming days.