The country has been in turmoil since a military coup in February, which sparked huge pro-democracy protests, a bloody crackdown and renewed fighting in ethnic border areas.

Clashes broke out in Mongko, Shan state, on August 28 when troops tried to seize a base from the Myanmar National Democratic Alliance Army (MNDAA), the group's spokesman and local media reports said.

"They came to take our base. They were hurt a lot as we were waiting at the top of the mountain and they were at the bottom. We shot them as they were coming," an MNDAA spokesman told AFP on condition of anonymity.

Fifteen soldiers were killed on August 28 and eight more in renewed clashes on September 1, he said, adding one MNDAA fighter had been killed.

AFP has not been able to verify the casualty figures.

The military said Monday an officer and an undisclosed number of personnel from other ranks were killed in fighting with the MNDAA in Mongko on August 28.

It did not respond to questions on the alleged later clashes.

Myanmar has more than 20 ethnic rebel groups, many of which hold territory in the country's border regions.

A messy struggle over autonomy, as well as control of lucrative drug production and natural resources, has long pitted them against each other and the military.

Video published last month by Chinese state media CGTN claimed to show the military fighting the Kachin Independence Army -- another group that operates in northern Shan state -- and "several other ethnic armed groups".

Stray bullets from the clash had landed in China and caused "damage to some buildings and fear among local residents", it added.

Many rebel groups in Myanmar's north share close cultural ties with China, speaking Chinese dialects and using the country's yuan currency.

In 2017, months of fighting between Myanmar's army and ethnic insurgents in the Kokang border region claimed dozens of lives and sent thousands fleeing from their homes -- many to China.