At least 19 people, including four members of the security forces, were killed by an armed group near the main border gate with China, a spokesperson for the Myanmar government told reporters.
The Ta'ang National Liberation Army (TNLA) launched three attacks on early Saturday in the town of Muse in Shan state, also injuring 20 others, Zaw Htay, the spokesperson said.
Mai Aik Kyaw, TNLA's spokesman, told AFP news agency that they attacked military posts, a casino just outside the town of Muse on a road to Lashio.
The TNLA and other ethnic groups have been fighting against government forces for decades, seeking greater autonomy and federalism.
Civilian leader Aung San Suu Kyi said ending Myanmar's long history of clashes was her main priority after she took power in 2016, but she shares power with the military that fought the insurgencies for decades.
More than a third of Myanmar's townships are affected by unresolved conflict, according to a 2017 report from the Asia Foundation.
Aung San Suu Kyi managed to bring two ethnic groups into a ceasefire accord in February, adding to eight others who had signed the deal before she took office.
