Burma has accused ethnic Kachin rebels of starting deadly fighting that erupted near China's border this month.
Officials told state media the army had to act after the Kachin Independence Army (KIA) ignored an order to withdraw from an area near a hydropower plant.
The fighting was the heaviest between Kachin rebels and government troops since they signed a truce 17 years ago.
The Kachin blame the government for the clashes, which have reportedly forced 10,000 people to flee their homes.
The government's accusation in the official newspaper the New Light of Myanmar is its first comment on the conflict.
It said the only objective of the army was "to protect its members and an important hydropower project of the nation".
The Tarpein dam is being built by China to help it meet its power needs.
But the KIA are opposed to the project, saying the dam will be environmentally and socially destructive.
Fighting broke out in the area on 9 July - at least four rebel fighters and a number of government troops have died. The rebels say they also destroyed several bridges.
Rights groups say some 10,000 people have been fled, many of them fearing being forcibly recruited as porters by the army.
The KIA is one of several militias aligned with Burma's ethnic minority groups who for decades fought against the central government. They are demanding greater autonomy for Kachin state.
Ahead of the country's nominally democratic elections last November - the first in two decades - the authorities proposed a ceasefire deal to the militia on the condition that they disarm and join a centralised border guard force.
The KIA refused to do so, resulting in the collapse of its long-standing truce with the government.
Neighbouring China has urged both sides to "resolve their differences through peaceful negotiations".
The US state department has called on Burmese government to "cease any such hostilities and begin a dialogue with opposition and ethnic minority groups towards national reconciliation".