The conflict, including in the northern-most state of Kachin, has escalated over the past month as the global focus remains on the Rohingya crisis in the west of the country.
The army has been separately accused of systematic human rights violations against the Rohingya Muslim minority in the western state of Rakhine, forcing more than 700,000 Rohingya to flee to neighbouring Bangladesh since last August.
Ethnic insurgents in Myanmar killed 19 people, including four members of the security forces, in an attack on security posts and a casino near the border with China, a Myanmar government spokesman said on Saturday.
Suu Kyi managed to bring two ethnic groups into a ceasefire accord in February, adding to eight others who had inked the deal before she took office.
Among the dead were 15 civilians, three soldiers and a police officer, he said. Video on social media showed smoke rising above Muse's prominent trading centre.
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Zaw Htay, the main Myanmar government spokesman, said around 100 insurgents attacked at about 5 a.m. using small arms and artillery, and were repelled by armed police and members of a government-backed militia he called "paramilitaries".
The KIA, one of Myanmar's most powerful ethnic minority armed groups, based in the mountains bordering China and India, has regularly clashed with the Myanmar army since a ceasefire broke down in 2011.
"Now the Myanmar military is attacking and following them. It is just a destructive terrorist act". It has to conduct minor military operations due to the deployment of large number of troops by Tatmadaw and intensified offensive attacks on its ally, the Kachin Independence Army (KIA).
More than 100,000 displaced people now reside in camps in Kachin and Shan states since a ceasefire between the KIA the military broke down in 2011, according to the latest United Nations statistics.
Myanmar's patchwork of ethnic groups make up around a third of the population.