Clashes between the Myanmar junta and armed ethnic groups have broken out again near a vital trade hub close to Thailand's border, an armed group spokesperson and the Thai army said Saturday, following a week of relative calm.

Myanmar has been roiled in conflict since the military seized power from a democratically elected government in a 2021 coup.

Fierce fighting broke out around the Myanmar trade hub Myawaddy earlier this month with the military forced to withdraw from its positions in the town.

However the area had been mostly quiet in the past week with locals reporting hearing no fighting, although a regional bloc expressed "deep concern" over the ongoing situation on Friday.

During the earlier clashes some soldiers had taken shelter under a bridge connecting Myadwaddy to the Thai town of Mae Sot, according to the anti-junta Karen National Union (KNU).

KNU spokesperson Padoh Saw Taw Nee confirmed to AFP that the group was fighting against the junta in Myawaddy, but did not give any further details.

Early Saturday morning, a Thai army unit stationed on the border said clashes were ongoing with a force targetting the junta soldiers under the bridge.

"Currently in the middle of fighting, no losses known," the Rajamnu Special Task Force posted on Facebook.

It said forces had deployed "drones" against soldiers under the 2nd Friendship Bridge -- a major trade point for the cash-strapped Myanmar military.

The Thai task force also confirmed the bridge was closed, and said its soldiers were standing by.

In a tweet, Thai Foreign Minister Parnpree Bahiddha-Nukara said they were closely monitoring the situation following reports of clashes Friday night.

The decision to pull troops from posts in the trading town was a major blow to the junta, which has suffered a string of losses in recent months.

Myawaddy is particularly important to the military, with more than $1.1 billion worth of trade passing through it in the 12 months to April.

A Myawaddy truck driver, who declined to give their name, told AFP they had heard clashes.

"Fighting is not in the town but outside of the town. We haven't heard anything at this moment," they said.

Local Thai media shared videos early Saturday morning of apparently scores of people queuing to cross into Thailand.