An alliance of three ethnic armed groups in Myanmar says more than 2,300 personnel of the country's military have surrendered as the two sides continue fighting in the country's east.

The Three Brotherhood Alliance has been stepping up its offensive against the military in Shan state since late October.

The alliance said on Friday that a military force of the Kokang minority took control of an area near the border with China.

It said 2,389 soldiers from the military's largest regional operational command laid down their arms, including six brigadier generals. About 1,600 family members are also said to have surrendered.

Local media are showing footage of the soldiers, believed to have been taken by the ethnic groups.

A spokesperson for Myanmar's military told local media that it is considering the security of the families of the military forces as the first priority.

The capitulation by junta troops is believed to be the largest surrender to ethnic militants since the groups began their coordinated offensives. It suggests morale among the military is falling as the junta struggles to maintain control.